If you're still having the problem after following Zylstra's advice:
"Instead of using Safe Mode, try Last Known Good Configuration with the drive removed"
then from the "Windows Advanced Options Menu"; if the following item is listed, select it. It should stop the automatic restart and probably show a Blue Screen of Death (BSOD) with an error message. Copy the entire message in it's entirety, post it here, or search internet for solution.
Disable The Automatic Restart On System Failure
The above Advanced Option menu item should appear for a Windows XP SP2 installation. I'm not sure if it appears on Windows XP orginal, or SP1 installation.