First I would try fixing the master boot record (MBR).
If you have your XP disc boot from it and press "R" when prompted to repair an exisitng installation using the recovery console. This should lead you to a DOS-like screen where you need to select which installation you wish to repair and enter the administrator password. Then you can type "fixmbr".
There is some more information
here.
A quick google for "fixmbr", "recovery console" etc. should throw up some more detailed instructions if needed.
Hope that helps.