Boot from Windows XP CD and let all file load, then press R and you will get access to Command prompt type window. In it enter following two commands -
1. Fixboot
2. Fixmbr
then restart the PC and see what happens, you can also check whether ntdetect.com is present in drive C of your PC, if not then download NTDetect.com and place it in C:\ with the help of Linux LIVE CD or any thing else like Windows PE.