What is your card. Read this. It sounds an unlikely scenario but who knows. (ATI)
When trying to install graphics cards ATI Radeon HD 3870x2 ou4870x2 specifically on computers with Windows Operating system, you may however be confronted with black screen displayed on your monitor during installation of catalyst drivers
According to ATI, you are required to wait for about twenty minutes till the hard drive stops working. You should now restart your computer and by pressing the reset buttons or by pressing and maintaining directly the start buttons to install the drivers properly.
Unplugging the monitor and restarting can also correct it sometimes.