don't reinstall, it can be fixed
put in the XP CD on boot, go to recovery and type del C:\boot.ini
then type edit.com (which is a DOS text editor) and make a new boot.ini with this in it:
[boot loader]
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Home Edition/Professional" /fastdetect