|Do you have your XP Install CD? If so boot to it. You will need to set bios boot order to CD first in order to do so.|
Once the install disk boots enter "R" for Recovery console.
Setup should detect the existing XP installation and prompt you for your Admin password in order to login.
Once logged in there are several things you can try to fix your boot issue. First is to repair the bootfiles by using the commands "FIXBOOT" followed by "FIXMBR"
If these do not work you can try CHKDISK.
It is also possible to use System Restore from within recovery console RSTRUI should get this to start.
I would try these and if they fail then you can always try a repair install.
Goin' Fishin' (Some day)