shaky,
Have you added any RAM lately or done anything else inside your case?
(I would make sure the ribbon is fully attacked on both ends. Mobo and Hard Drive.)
If you have not added any RAM and the ribbon is on the Hard Drive securely, I would boot with the WinXP CD and go into Recovery Console. In there I would run FixMBR:
fixmbr device name
(In this care device name would be the drive letter of your Hard Drive.)
Take Care,
Richard