Answer:
There can be a few problems that cause this. First things first, make sure you've run an offline chkdsk. The boot loader files could have filesystem-related errors.
One possible problem could be that the entries in boot.ini are wrong. This can be fixed by editing your boot.ini file and checking for inconsistencies (often the wrong drive controller and disk is selected). Another likely problem could be that your boot files are corrupt. Try booting to the Windows Recovery Console and running "fixboot".
If you do not understand or are not comfortable making these changes yourself, have a professional look at it. You could turn a $20 problem in to a $200 problem.