The best solution is clean Windows installation.

replug all cables, check temps, what psu? e84 ocd? swap ram sticks.

Assuming u got 3 1gb ram sticks, fault could be one of those failed. Honestly no idea if a failing ram stick would cause displaced pixels and visual noise, but it will cause pc to be slower if it'll run with it. I recently gave my mothers husband's pc new ram 4GB up from 2GB and it made a huge difference in speed (also minimized 'passive' taskbar stuff he had running for no real reason). If possible, I would try to get my hands on some ram and test if i were you.

The best solution is clean Windows installation.

He tried that already.... OP I would first test ram and HDD with Memtest86 and an app like HD Tune, just to check. Artifacts (bad pixels) usually indicate a hardware fault. If you are able to get a screenshot of the graphics corruption, that will help to determine the cause.... Is there anything helpful in Event Log?

Like pill monster says sounds like hdd or ram slowing you down. My bet is on the ram since you have swapped gpu and you are getting artifacts. How is the cpu temp? Sometimes that could cause degraded windows performance if its over heating but i do not think that is causing the artifacts!