Re-install Windows from the recovery discs or from the recovery partition.
If recovery or re-install doesn't work, test the hard drive to start with,
using DOS-based diagnostic software from the drive manufacturer's website. The links are here:
http://www.tacktech.com/display.cfm?ttid=287
Create a CD from the downloaded ISO file on a working computer using
IMGBurn:
http://www.filehippo.com/download_imgburn/
Boot your faulty laptop from that CD and the diagnostic program will eventually open.
Test the laptop's hard drive with the Long/Extended test.