F12
Turn the computer on and when the DELL logo appears press F12
this will get you into Diagnostics where you can test the display. If it is a malware problem it will go way while in diagnostics, if it is a hardware problem it will stay.
I don't know if all Dells have diagnostics built into BIOS, but that sounds like the best suggestion I've seen on this thread. Another good suggestion is running a Linux demo CD. If both of those work fine, then I would consider that proof Windows has been infected.