After you have gone to http://www.solidworks.com/sw/support/videocardtesting.html and downloaded your correct, approved, tested, video driver for your card to run SolidWorks correctly you need to keep it from changing or updating.
Windows 7 wants to install the latest drivers from Windows Update by default. This is great for your mouse, keyboard, printer, etc., but not for your graphics card.
To change this: right click on Computer in the start menu, click properties and choose advanced system settings from the side menu bar, choose the hardware tab and click the Device installation settings.
This will bring up a menu with the question: Do you want Windows to download software and realistic icons for your devices?
Choose the option: No, let me choose what to do.
A new set of choices will drop down, if you choose: Install driver software from Windows update if it is not found on my computer it will only use windows update as a last resort for finding an appropriate driver.
See web resource http://community.spiceworks.com/how_to/show/2057 for basis of this information.