I recently brought my computer back from my friend's house after playing some games. When I turned it on today, it would go to the loading screen, and then the monitor would just go idle (Power light goes from green to yellow) before reaching the log-in screen. I don't really know what the problem is since it has been running fine until today. I tried booting from another OS from one of 2 other hard drives, and it worked fine.

I tried going through safe mode, but a wall of text would appear and just stay there for a long time. I also tried the option which goes back to the last time it worked smoothly, but my monitor would just go idle again.

What is the problem and how can I fix this?

Maybe the PC switched video ports? So plug the monitor to a different port?
It might still be loading?

That was the first thing I tried, and it didn't work.

Anyways, I'm using Windows XP and the message I get on my LCD screen is "No Signal Input".

bad video drivers?
It does that. Wait for 5 minutes, just to be sure it's really stuck.
If you really can't get into safe mode the something is terribly wrong and you might want to consider reinstalling windows.
If you get into safe mode after sufficient wait time, try to delete your monitor and graphics card drivers.
Then it should boot back in normal mode with a low resolution and you can reinstall the drivers.

Bad HDD, bud. Monitor works fine, right? OS on HDD 1 won't boot, since it gets stuck at something, but OS on HDD 2 and 3 work fine?

Boot into those OSes, backup, run a disk scanning program, then possibly reinstall the OS.

The problem is that I can only log in as guest. I forgot my password since I used that HD's OS 3 years ago.

If you do have a hard drive problem, like bad sectors or whatever, even if the disk utility is able to 'repair' the drive and everything seems to work fine, replace the drive ASAP and backup any data that is currently on it. Be aware that the more you use the drive when it is in a bad state, the more likely it is to die - so backup your most important files first, and don't do anything that'll thrash the drive.

This recently happened to an old computer of my dad's - developed bad sectors and wouldn't boot into windows. After running disk-check on it, it repaired the bad sectors and the computer was able to boot up. We bought a new HD and installed windows onto that. A few weeks later the original drive (which dad was now using as a 2nd backup drive) died completely.

Long story short: if a hard drive develops any kind of problem, replace it, because it is likely going to die soon. Treat hard drive problems as helpful warnings of imminent data loss, because often you don't get any warning at all.

Usually I despise "I agree" posts that do not add anything, but I feel this is very good and comprehensive advice about that kind of problem, so I want to stress that you take it to heart.

Actually, I got one thing to add: A technical explanation.
HDDs have a good reserve of spare sectors that work as backup should one of the sectors of the main part of the disk fail. The HDD automatically detects data corruption and reroutes to a spare sector. This normally doesn't lead to data corruption visible for the user. But as more and more sectors die out, the reserve is being used up and if none is left, you experience data loss. This means you already had several dozens of MBs that went bad without you noticing so the HDD is at the end of its lifetime.
So basically before you notice the first error there have already happened many many more and the frequency of errors will increase in an exponential fashion from now on. So don't think "Ah well, just 1 random error, who cares?" and backup ASAP.

As the others have stated, this is very likely a corrupt OS caused by a failing HDD.

Install new OS to new HDD and back up data from the old HDD to it before it fails completely.

This is going to be such a pain. I'm going to have to reset all of my paid apps on my iphone.

You could do a sector-to-sector copy of your system partition. If it's not too damaged you could get back into your old windows with a new drive.

Backup important data first before doing so, however, just in case heavy usage like that does cause the drive to fail.

I tried doing that, but I get an error saying that Back Up has failed.

I mean picking out the specific files/folders you want and copying them to something else.

That sounds like you don't know how to conduct the measures we recommended. If so, stop fooling around at once or your data might be lost for good.

For the backup I wouldn't use any backup software. Just copy whatever data you need over to a good drive (physical hdd, not partition!) or burn it on DVDs. That way you are in full control of what's happening and are informed about possible read errors so you can just leave out that part.

About the sector-to-sector copy you first need a program capable of doing this and an idea what you have to do. If you lack any of it, ask away.

Incidentally, I had a problem similar to this without any hardware failures involved, and it could possibly be the same issue you've run across here.

Windows XP is stupid and doesn't handle USB drivers very well. You have multiple ports on your computer, but XP treats each device on each port as a completely unique device. If you plug your mouse into port 1, it'll load the drivers for that mouse on port 1. If you move the mouse to port 2, it must re-load the drivers on port 2 before the mouse will work on port 2. The really dumb part is that XP can often have difficulty loading drivers when you've booted with your peripherals in a new configuration when it first boots up.

I *just* saw this with my boyfriend's computer this weekend - plugged in a friend's wireless mouse, and XP shows the loading screen, but goes black before it ever gets to the login screen. If you remove the mouse and reboot, it works fine. If you plug the mouse in after it has booted, it works fine. No hardware failures whatsoever, just XP being really lame with USB drivers at bootup.

Now the reason I suspect this may have happened in your case, is because you took your computer to you friend's house. Now that you've got it back, suddenly it's not working - perhaps you plugged the peripherals into different ports than what they were previously?

Also this is a common problem to happen when upgrading hardware. Say you buy a new graphics card, so you unplug everything from the back so you can move the computer somewhere easier to work on it, put in the new card and plug everything back in, and suddenly XP won't boot properly. I think you've fried something when you made the hardware change, so you drag it all out again and put the old graphics card back in, but you still plug the mouse back into the same port that you just had it, and again windows won't boot properly - this can be quite frustrating and scary if you think you just broke your computer.

Please note that I am not suggesting this is *definitely*, or even *likely* the problem you are having, but it is something to be aware of that can happen.

I need a new hard drive to install my OS.

If I'm correct, the two different types of hard drives are IDE and SATA right? Last time I made a mistake of getting SATA. The hard drive that broke down is Enhanced IDE Hard Drive and I want to get the same type.

Can someone recommend me a cheap one? I don't want anything big like 500+ GB. Something under 100 or even 50 would be good. Thanks.

Under $100 is all there is. Buy the one you like. All of them are pretty much the same, and this computer won't last you much longer, so there's no point in getting a superb quality hard drive. That being said, the caviar blues are probably the best, if they are anything like their SATA counterparts. I suggest you get the one which has the highest price/GB and is the max size you want. The 500 GB ones will most likely be the best deal.

What's wrong with SATA?

http://www.computer-hardware-explained.com/sata-vs-ide.html

The reason is because he would need an IDE to SATA adapter.
But still you got a point. It may be worth considering buying the adapter because if you computer still has IDE it's pretty old and will become obsolete in a few years and then there might not even be IDE boards anymore so you would have to trash your IDE drive which you could still use for several years if it was SATA.