Since you have done a (clean?) install of W7, that eliminates a lot of common problems.You can certainly stop thinking about viruses.
I have a couple of questions:
Where did you get the video driver(s) for the laptop when you did the reinstall?
From the HP website only, or did you use the driver package from ATI?
your video performance will drop, but does that make the artifacts and freezing stop?
This test is to try and isolate the problem graphics chip.
Does this behavior happen only on battery, or when the battery and power supply are both connected?
What happens if you run on the power supply only and remove the battery completely?
I didn't find the right solution from the Internet.
Construction Equipment Video