Hi! Today, at the beginning of my work, I saw that my PC reboot automatically as soon as WINXP is going to be loaded. It a P4 with 2x512 RAM, mother GIGBAYTE. In past days, however, it used to reboot occasionally, but after log in WINXP, but now before log into Windows, it rebbots. I tried to log into Safe mode but it reboots, too. Recently I've added no new hardware. Please give me your solution and any suggestion

This issue is normally caused by serious instability in your system.
It's quite worrying that you acn't access safe mode.
A repair install maybe your best course of action especially if you don't want to try a full rebuild.
Here's the official Micro$oft way of doing a repair install,
And Here's one with screen shots.

The good thing about this method is that it keeps all of your current settings.