Computer stuck on Windows loading screen?

My buddies computer is stuck on the windows loading screen (the one with the green loading bar at the bottom). it was working earlier but i tried to pull some stuff off MY old hard drive on his computer and it didnt work... since then it wont load the OS. F8 does nothing.. F11 only brings up Floppy, CD drive, and HDD boot options... and then F2 is just the bios. I tried removing the CMOS battery and it did nothing. Another buddy said it was a memory problem, but when it load up it says the 3528MB of memory is OK.. .also the comp. has 4GB of memory so its not showing the appropriate amount ( maybe the MOBOs max supported tho) any suggestions?