First boot into your BIOS, and select the option to load BIOS defaults. Reboot. Did that solve it? If not continue on.

Unplug the CDRW drive. Try rebooting. Did that fix your problem? Check your Ram? If you have two sticks, try taking one out, rebooting and see if that solves your problem, if not try the other.

Also check all cables, cards, and connectors. Make sure they are firmly seated and are working correctly.

You could try to repair the current installation, by popping your XP Cd in, booting to the CD and choosing the repair option. If that doesn't work, go on to the next suggestion.

You could always just put the windows CD in the drive, and try reinstalling over the existing installation. Just run through the windows setup and instead of formatting, leave the current file system intact and reinstall windows. That should correct any software issue that might be causing this.