boot from windows xp cd-rom & choose "repair" "R key" .. after booting type :
chkdsk c: /p
when it finish try to boot your system ,, if didn't work :
boot from a mini windows xp cd (like hiren boot 9.8 & later) or a windows xp PE CD , then copy your important data in the c: drive (desktop , documents , etc) to another disk or better to a flash drive
(Note:connect the flash drive before turning on the laptop to make sure mini windows sees it)
copy your data then reinstall windows xp !
I can't find a reson for what you said .. but I guess this is the fastest solution ..
If you need more instructions ... tell me & I'll write it step by step
but it will take some time to write