Hypothetically, lets say i buy Vista and activate it. In a few month's time i might want to reformat. Can i reformat my hard drive and fresh install Vista again, or will it only activate once?
I'm not sure about Vista, but at least in XP OEM you could activate via Internet twice without having to call Microsoft. The limit seems to reset in four months or so.
That's only my experiences with XP x64 - which is available only as an OEM version. I had to call them to activate it after I had installed it three times in a couple of months. I installed it again about five or six months after that, and had no problems activating via Internet.
Of course there are some speculations that Vista limits this more as well, but I don't know. I'd guess OEM versions have more restrictions than retail ones, anyway.
Besides, is there a reason you can't wait when there's one day left to activate?