If you have FAT32 partitions, it is much simpler than with NTFS. Just boot with a Win98 floppy and copy the NTLDR or NTDETECT.COM files from the i386 directory to the root of the C:\ drive.
For NTFS:
1.Insert and boot from your Windows XP CD.
2.At the first R=Repair option, press the R key
3.Press the number that corresponds to the correct location for the installation of Windows you want to repair. Typically this will be #1
4.Enter in the administrator password when requested
5.Enter in the following commands (X: is replaced by the actual drive letter that is assigned to the CD ROM drive)
*COPY X:\i386\NTLDR C\:
*COPY X:\i386\NTDETECT.COM C:\
6.Take out the CD and type exit