Just to note: You can actually do this from any Sense-based ROM - it doesn't need to be "stock" and it doesn't need to be unrooted. The important thing is the dialer app, because Sense's dialer has the special ##GPSCLRX# code in it and the AOSP dialer does not.
Probably the easiest thing to do is simply to get a Sense ROM and flash it like you normally would either via ROM Manager or manually via your recovery. There's a stock (rooted) 3.70 here, but you could also use something like Fresh 3.5 or MikFroyo if you prefer.
If you want to actually go back to a stock and fully unrooted ROM, then Matthew Read's answer is a good explanation which doesn't require Windows (or a PC at all, really).