Can't add my GeForce 8400M GT Graphics Card

So, I have upgraded my laptop from Vista-32bits to Windows 7-32bits. I often play games and once my graphics card failed and made my laptop restart. In my attempt to fix it I think I only made it worse by messing up in the device manager, it seems I deleted my graphics card to try and reinstall it. Now I don't seem to be able to re-add it to my device manager, I've tried to download the driver but it does't seem to work, I am now using the Standard VGA dislpay. I have a Sony Vaio VGN-NR21S. What can I do to get it back to work, is it a hardware problem or a driver problem? Thanks in advance.

Thank you so much! It worked Rep+

That solution worked for a while but now my screen is full of red stripes and just won't load Windows.
In other words now I can't even access my computer. I guess I'm into some kind of problem with the hardware then. Before I made this thread my laptop had red stripes all over the place too... But I still managed to use it, now I can't.