What is the brand/ model of computer?
Most store bought ones like Dell have recovery partitions or recovery cd's.
These will take you back to brand new. Keep in mind that ALL DATA WILL BE LOST if you do not back it up.
You will have to re-install all updates from microsoft.
You will have to re-install any programs you have bought or downloaded.
You will also have to do any custimization to the computer that you have done to get it the way you want.
With that said, are you have any particular problems that make you to want to do this?