Check the BIOS and see if there is a hardware scan utility, scan the hard drive for errors. If it finds some, get a new hard drive, install Windows on that and copy your files from the original drive to the new setup.
If the disk is OK, and repair fails, boot off your Windows disk and try a Repair Setup, if that does not work either, try running one RAM stick at a time, remove any add-on parts you don't absolutely need like video card if there is one aside from onboard video, wifi card, etc... and see if it boots.
If that also does not help, you should do a clean Windows setup. I always suggest getting a new hard drive and doing a clean setup on that to make sure your don't lose any files from the original setup.