Depending on your situation, below are some things to consider
NTLDR is missing
Simple "ntldr is missing" fix with boot floppy, CD-R, or USB flash
Lot of reading above, you can try the steps above or you can do a a repair installation -
Repair Installation
A Repair Install will replace the system files with the files on the XP CD used for the Repair Install. It will leave your applications and settings intact, but Windows updates will need to be reapplied.
Perform a Repair Installation