Keyboard/mouse not working after POST

I am working on a Dell Studio XPS 7100 that appears to have Windows 7 Ultimate installed. I tried to set the boot sequence in BIOS to boot from DVD as I think there is a virus in the system. However, it keeps going straight to the Windows logon screen. While in BIOS the keyboard works fine. However, once I am in the Windows logon page niether the keyboard nor the mouse works.

Any ideas on what the problem might be and how to resolve it. I suspect the computer has a lot of problems, but I can't get in to work on it.

thanks.