I had the same problem on one of my laptops - here is how I sorted it out
Get hold of the Windows XP Install CD and start the device using the CD-Rom. At the prompt, select the Recovery Console option, select the hard drive you want to scan then have your administrator's password at hand. Once you arrive at the prompt c:\windows, type cd system32 [enter] then type the command chkdsk /r. This will check the integrity of the hard disk and its files. It might take a while but it will eventually finish - restart the machine and there you are...