It sounds like the video card was not the problem,so there is a whole new issue. Until you get this resolved, please put the original card back in the problem machine.
By the way, did you uninstall the previous video card drivers before you made the swap?
So, with the problem machine and it's original video card what happens exactly - any error messages,etc. When did this start? Details are important.
Do you have a real Windows CD to reinstall if necessary?