do you have your product key? if not, google windows 7 product key finder, get your product key, write it down, go to digital river, download the windows 7 iso (it's legal) and either burn the disc, or use the Microsoft DVD/USB tool (download this too) and make a bootable USB key with the image, install, use the product key extracted in the first step, good to go.
You should also download your drivers too, or at least your network drivers. Been a few times I reinstalled my only PC at the time, and windows didn't find network drivers, which you need to get other drivers or do anything online with. lol. Pisser when you're calling a friend at 3am asking to come over and download some files. lol.