Video trouble

My wife just upgraded her Parallels. I started another thread about her computer being slower with her XP VM. But she has another trouble. She only uses full screen mode. Often when she starts the new Parallels, her screen is filled up with garbage. I found we can get out of it by moving the cursor to the top right where I can switch Parallels mode. When I switch back to full screen, she can play her games.

I bumped video RAM from 128 to 192 and regular RAM from 636 to 800 and now the funny video is different, and the solution is the same.

How do we fix this problem so it doesn't happen again?