Wow so no one really knows how to answer this. First of all, if your computer can't boot into windows, thank God it still turns on.
Second, you can do a repair on the sytem by using a bootable windows disc (XP users need XP and Vista needs Vista). Boot the cd first, then instead of doing a fresh install, choose "R" for repair. Next screen you will need to specify the partition (normally its 1) and the password if applicable. Next type in chkdsk /f and let it run and finish. Next try the sfc /scannow and if it runs great, if not, restart your computer by typing exit on the next line and boot off the hard drive
If that doesn't work then download a copy of WinPE (for vista and above) or BartPE (for XP and below). You can find them on Google, search for "WinPE" (put in the quotes for exact match) and "BartPE". These are pre-environment CDs that allow you to view your hard drive without actually having a operating system. Download the file as .iso and use a program like poweriso or imgburn to burn the image to the disc. Put the disc in your crappy computer and select to boot off cd. Save your files.
I'd recommend if you can't boot into windows even after a fresh installation, you need to replace the hard drive (if it makes a clicking sound), ram, bios chip, power supply, motherboard.