Did you install anything recently ... such as software for a new external hard drive or some kind of network device? I'm wondering if it's a driver conflict that's preventing Windows from booting.
Can you boot into Windows safe mode? (Usually done by pressing f8 while booting.)
If so, it's almost certainly a driver issue and you'll need to go into Windows device manager and uninstall some drivers.
If you can boot into Safe Mode but can't figure out which driver is to blame, then I'd recommend using a Windows installation disc (not the recovery disc that came with your PC but an actual Windows installation disc and start a fresh installation process ... with a catch. If your original Windows install is remotely intact it should ask you to Repair your Windows XP/Vista/7 installation instead of deleting the Windows directory. As long as the repair option shows up then it will repair your Windows installation and keep all of your settings and files.