Difficult. there are several things that may do this. If you are unsure about it the simplest first step is to try a reinstall of windows.
1> Is it XP?, try booting in VGA mode (hold down F5 on boot up), this will bypass the video card, corrupt drivers or a failed card can cause this.
2.My next check would be on the physical memory failing , If you can boot it and get into the bios setup (usually F2 or DEL), turn off SILENT BOOT, the bios will run a quick memory scan on bootup which may throw up an error. Failing that download a standalone memory test tool. (sorry link deleted, no external links allowed)
You have to burn this to a boot CD, there are lots of others that run off floppy drives.
3. Failed motherboard. Needs a hard test with a diag board, you will need a PC bloke to do this.
4. The other thing you can do is physically unplug the hard drive and see if it boots up, At least this will prove the hardware works (apart from the memory under stress). As it could simply be a corrupt windows that needs reinstalling. Possibly a virus.
In my opinion is not a hardware issue, if the motherboard had failed it would have not booted at all just a dead screen, I know it has happened to me before.
I would be happy to guess at its some corrupt drivers or something with windows xp.
What was the last thing you installed on the system?
I would make an attempt to enter in safe mode, do this by keep tapping F8 when the pc boots up (usually just after the BIOS loading screen). It will give you a list just choose safe mode for the time being without anything else.
If it manages to get into windows and you can perform a few simple tasks that would suggest to me something has gone wrong with your xp install. Two options then either format the disk drive or do a system rollback (if you have enabled it)
If you have any data you need/wish to keep do NOT format the drive you will lose all the data. Instead take the hard drive out put it into another PC and copy the data onto something else first. Then feel free to format the drive.
If you need more info on how to format the drive just let me know.
If mine I would put a Lixux live DVD in the drive and see if it will boot from that. i.e. Not using anything from micro-soft I got DVD with a mag very handy allows one to get stuff when hard drive US not all of course but at least you have not lost it all.