Didn't you get a CD when you bought it, or some other method such as ascessing a separate partiton on you HDD where the instalation files are stored. Check the documentation that came with your machine.
Without either of these methods there is no way unless you purchase a retail (NOT OEM) copy.
You will also need drivers for your video, sound, chipset, and anything else that may have come with it.
Really you should have thought about all of this before wiping the machine.