If you get an error that NTLDR is not found during bootup,
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 WindowsXP 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 ROM and type exit