Sounds like screen corruption, possibly from bad memory on the video card. (I doubt it's the main memory that's corrupt, or the PC will be blue screening a lot.) Perhaps try replacing the card, or testing the card in another machine.

As for the computer slowing down, that's kinda normal with Windows as you install more and more stuff If he thinks it's slowing down way too much compared to stuff he's adding though, try doing a virus scan and spyware scan