Your computer would have come with instructions on how to do that, it varies by vendor. Some use a partition you can boot from when the PC starts (will say "Hit Key Blah Blah to start setup" or something), some come with DVDs to do a restore. Some want you to create the DVD disks using a utility installed on the computer. No single way. You can also just buy a new Win 7 disk, collect the drivers for your hardware, and install it yourself.