Drivers most likely. Pop your card back in and before the Windows loading screen comes up, hit F8. You should see a list of different boot options. Boot up in Safe Mode with Networking. Now what you want to do is clean out your old drivers. Use <a href="http://www.drivercleaner.net/">Driver Cleaner Pro</a> (it's free), find your Intel card under "Cleaning Selection" and click "Start". Now you want to download the newest Nvidia drivers. Go to http://www.nvidia.com/object/winxp_2k_71.89.html and download the drivers. Install them, restart your machine and see if it'll boot up into Windows normally.