Boot to your Windows XP disk and select R to access the recovery console.
"to repair a windows XP....press R"
Enter the administrator password (blank by default).
Type these commands in:
COPY X:\i386\NTLDR C\:
COPY X:\i386\NTDETECT.COM C:\
where X is the drive letter for your CD drive.
Remove the CD and type exit.
Reboot your machine and see if that has fixed the problem.