Well, hard situation... Ok, let's try to handle with it:
1) First thing you will need to do, IMHO, is to replace your new Radeon with your old GeForce.
2) Windows should start normally. Perhaps, you will need to install GeForce driver again.
3) Make sure that Windows starts and works normally. Then uninstall GeForce driver and remove GeForce card itself. Maybe after that you will need to use some registry cleaner program like
CCleaner.
4) Now you may install your new Radeon card.
Good luck!