Blender 2.66 & GPU > after update

Yesterday I’ve updated my GPU. It’s a Nvidia GeForce GT620.
Today i’ve opened a blender files and immediatly I realized that something was wrong.
Before the updating of the GPU all was perfect but now Blender i very slow, the various option windows doesn’t open and when I try to do a render with the GPU it crash.
With the CPU the render doesn’t create any problem but blender sitll be very slow.

I thought ‘Fantastic, a new update for my GPU!’ but in this case it doesn’t seem to have been fantastic.
What can I do? Is unistall the update better for me?

Sorry if there are some wrong words or phrases, i’m Italian ^^

PS: my PC is a Dell Inspiron 660, i5, w8 64-bit.

Hey man I just made a post about this, but to resolve the issue for now download/install a graphics driver earlier than the latest version, I was having issues with my GF GT 640 update: 314.07-desktop-win8-win7-winvista-64bit-english-whql.exe I downloaded a previous version and everything is working as it was, hope this helps bro!