Can you please edit out all the blank space at the bottom of your post? It makes it hard to refer back to it while typing a response.
Without further testing there's no way to tell if it's a video card or not. You may want to check with the PC shops that you used to see if they tried another video card. (I'd suspect that they would have, and if so this may save you buying a new video card). I'd tend to trust the PC shop's word - since they have nothing to gain by telling you they can't fix it.
I suspect that it's not the video card - otherwise you probably wouldn't be able to ever get into the system (although there are some uncommon problems with video cards that could cause this).
The first place to check is in your Event Viewer for error messages. If there are no errors, then that's a sign that the problem is occurring below the level that Windows can recognize. Here's a link about how to use the Event Viewer. It's slanted towards the Blue Screen of Death - but the principles remain the same: http://www.bleepingcomputer.com/forums/t/40108/how-to-use-event-viewer/
Let us know what errors you find and we can work from there.
OH! Another thought! Try booting into Safe Mode and see if that works any better. That'll help to confirm/deny a driver problem. You can get to Safe Mode by repeatedly tapping the F8 key at the moment that Windows begins to load (not the initial BIOS screens).
My browser caused a flood of traffic, sio my IP address was banned. Hope to fix it soon. Will get back to posting as soon as Im able. - John
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