I used to have 2 gtx 285 in SLI over clocked. After a while my water pump died and cards were getting really hot and those things started to get left over. Even after cooling problems were fixed those artefacts were left behind until I changed graphics. With new cards those things disappeared.
Weird thing is it only happened with start menu and tool bar related stuff.
You might want to check if you have latest GPU drivers and use some GPU testing software.
