It doesn't just clear the registry. It clears the entire partition. I should explain that hard disks can be partitioned into many parts so one physical disk can contain many logical disks. If you reformat your boot drive - usually drive C: - you will lose everything on it; the registry, your documents, your operating system, the lot. Next time you switch your computer on you will get a meesage along the lines of "Non-system disk or disk error" This is BIOS speak for "Where's the b****y operating system gone?" You will have to reboot from a floppy disk or CD and reinstall your operating system. Is this what you want?
Just for the record, reformatting doesn't actually delete much. It just marks the space as empty. The same thing happens when you 'delete' a file. If you really want to clean a disk - which is always advisable if it came from an unknown source and you don't want to be Gary Glittered - do an UNCONDITIONAL format. Boot from a DOS floppy and enter FORMAT C: /U.