@Shalu_Sharma
yes, what satyamy wrote will show u whether u really have the file missing or not, and suppose if u realy have the file missing, then go to start,
RUN, and type
SFC /SCANNOW, System File Checker utlity will check for any missing or curropt file to restore it back
Note for this command, u will need to have the Windows XP, installation CD or Source in hand