After installation, Windows won't start up.

I had windows 7 installed on my machine and because of a virus infection, I had to format my C: drive and reinstall the operating system.

The disc seems to install windows 7 fine, but then when I go to start windows, nothing happens. The 'starting windows screen' appears but then it just leaves a blank black screen with the mouse cursor.

I tried again with all external usb drives/cams taken out and also ran a memory diagnostic, but it didn't help.

What could be the problem?

Leaving your USB devices hooked to your computer during a clean install messed up the install. Reinstall without any USB or eSATA devices plugged in If your USB hard drive was on that's probably where Windows got installed or the Master Boot Record was messed up due to USB devices being plugged into USB ports during installation of Windows.