well can you get to the advanced boot options menu? (to do this while the computer is starting up hit F8 till it shows, must be done right away after power on)
If not put your windows CD and boot from it and goto the recovery console. You will have to select a partion likely C:, and then enter the admin password which is likely nothing.
once you get the command prompt type chkdsk /r this will check for corruption to the hard drive. If that doesn't work, then go back to the recovery console and try the command fixboot. If those don't work it still may be possible to fix it, but you would then need access to a external hard drive enclosure and a second computer