I would try a new CMOS battery first like Tech_Guru suggested.
If that doesn't help, I'd then try another known working power supply.
If that doesn't help then I'd suspect the hardrive next. The hard drive you ask? Yes. I've seen damaged hard drives create strange problems even before booting starts. Try another hard drive in your machine. I know that it won't necessarily boot smoothly (especially with Win XP), but if it passes the initial problems of pressing F5 etc. then you know the source of the problem.
If the hard drive test still doesn't resolve the problem then it may be the motherboard and you may have to bit the bullet and get a new one.
Whatever the source of the problem whether it's power supply, RAM, hard drive, mother board, etc. remember that you'd better run out and buy a replacement before May 1st!