Seems very likely that your installation of XP is corrupt, if it can't find a system file it needs then it will reboot so if the file is missing or corrupt then it will just keep on rebooting over and over, if you have got one, put in the Windows XP CD, boot from it and after a while you should get an option come up that says 'Repair using recovery console' choose this which will then take you to a black screen, then type 'chkdsk' this should help, if not you might need to reinstall XP
today i turned on the pc and just before the welcome screen should appear the pc decides to reboot and then keep on rebooting.
i tried in safe mode and it still rebooted. i checked setup at the beginning and no obvious problems. been fine and then all of a sudden doesn't boot.
any help much appreciated.
the system is:
Antec SLK2600 350w - Asus A7V8X-X -Athlon XP2800+ 'Barton' - 512Mb TwinMos PC2700 DDR - Ati Radeon 9800SE @ stock - Maxtor 80Gb 7,200RPM 8MB EIDE - DVD-ROM/CDRW - WINXP Pro SP2