What is the make and model of your desktop pc? Most of them have a hidden 'restore to factory settings' partition. Or a program that runs to make a restore DVD. Using those will preserve the oem pre-activated Windows 7 and you don't need to type in a product key or worry about the correct drivers.
If you do decide to reinstall using a DVD you will have to use the product key on the sticker on the case and that will require telephone activation with MS
You can download and burn your own DVD from the official MS .iso file link on Heidoc.net scroll down the page to the English(USA) version, get the SP1 media refresh as that already has SP1 on it and will save time and hassle when updating.