If you look at your drives, you should have a drive named OS Tools. On this is a mirror or ghost image of your system along with several importent utilities. Don't mess with it unless you know what you doing or are developing something.
That said, you can and should make yourself a back up and/or a set of recovery(system repair) disks. Control panel>backup and restore>create a system repair disk.
You can also always go back to the original "clean slate" condition as Coaledsweat suggested, but that would only be if you were reselling the unit to someone else because obviously you lose any and all info.