hmm last time I went from ATI to nvidia (or was it nvidia to ATI?) I had to format to get games to stop crashing, windows still booted though.
Have you tried a simple safemode?
Maybe the ATI uninstaller accidentally took out some key windows files.
If you can't fix windows put ubuntu on a usb stick with this:
http://unetbootin.sourceforge.net/
Copy all your data off onto a usb hard drive or burn it or put it on another partition & just format.