this usually happens because the boot.ini is corrupt
go into the recovery console (startup from the XP CD and press 'r')
then type "edit.com C:\boot.ini"
now you will be able to edit the boot.ini from a DOS text editor
you will need to put this into the boot.ini
multi(0)disk(0)rdisk(0)partition(1)\Windows="Micro soft Windows XP Professional" /fastdetect
then save, and reboot
if you can't edit it because it's read-only, make a new one called boot1.ini, put the above in it, save and exit, then when you're back to the recovery console, type "copy C:\boot1.ini C:\boot.ini"
this will overwrite the boot.ini with the boot1.ini you've just made