Do you have any isntallation CD?

Just put the window installtion CD, not recovery CD, then pretend to install windows, at the step to ask you where to install, then you can select to delete the original system parition, you can delete all paritions you see, then format the hard drive.

Or if you have another computer, u may open the case to take the hard drive and connect to another computer as user disk, then you can format it from the disk manager (from the control panel you can find it. -> administrative tools -> computer management ->disk manager))