sorry i have no idea but i would try running the system file checker, click 'start/run' then type 'sfc /scannow' and click ok. insert your windows disc if prompted.
you wont be prompted for a disc if windows was pre installed on your computer. this will check the windows system files and replace any missing or damaged ones. it wont do any harm to check this. its all i can think of because i cannot find any info sorry.
ps. if its only started to happen recently then try system restore to when it was all ok maybe.
and/or try a visit to windows update click here and check that xp is up to date.