This is almost always a sign of corrupt files due to hardware malfunction. Sometimes, it's a virus. A complete hard drive crash wouldn't get you past the BIOS post. And, if it was an overheating cpu, you would be able to boot up when it's cold.
Try this. Put in your XP cd and boot up. (CD drive must be first in the boot order in your BIOS). Proceed as if you are doing an install. Eventually, you will reach a point where you can press R for repair. Do so; and you will be at a prompt. Type in fixmbr; and then type in fixboot when you get the cursor back. Take out the cd and try to get to your desktop.
If that didn't do it for you, go to a friend's house to use his machine where I urge you to get SpinRite 6.0 ($$) from grc.com; make a floppy, and run it at level4 on your comp to try to recover the data on the hd. (I hope you have a floppy drive). If you don't have a floppy drive, you can burn a cd that will run the program. This will take 8-15 hours depending on cpu speed and drive size. Once in a while, check the Detailed Technical Log to see if there are entries of problems. If the SpinRite test shows the drive to be okay, and it still doesn't boot up, then you need to take your hd to a friend's house; hook it up to his main drive as a Slave (it temporarily becomes a "D" drive then) and check it for viruses.
File corruption can be caused by power supplies that are starting to fail; also, hard drives that start developing bad spots. Less likely, but possible, is a failing memory stick. And finally, though remote, is the occasional failing mobo.
Another thought, while this lovely "friend" of yours is allowing you time on his (or her) computer, go to your machines' web site and see if there is a BIOS update. These are accomplished with a floppy as well.
Good luck!