Sometimes if you activate Windows too many times you'll be forced to call in every time after to activate... I have to do it every time I format now -- calling and talking to a person in India just so they can give me a new key.

You are allowed to have the copy of windows that you purchased on one pc at a time. For example, if you owned two computers and one copy of Vista, you can't install that copy on both machines unless you purchased a multi user license. Only a business would do something like this though, as the multi user licenses are sold in blocks, i.e a 10 user license or 50 user license. Those numbers are just as an example. I don't know what microsft's blocks are currently. So, for most home users and that includes your situation with one PC, there's no issue of you breaking the End User Licensing Agreement by re installing your Vista copy on the machine you're taking it off. It's doubtful that you'll have to call in to activate it for the first re install, at least with XP you didn't until the third, I believe. You ought to be able to do it over the net after your fresh installation. The only thing that would change this is if you've radically altered the computer between the time you first activated Vista and now. Any new network cards, new CPU or new motherboard will force a phone call activation. It's no huge issue, just have the number that you see on the screen at the failed activation prompt handy when you call Microsoft so they can issue you a new activation code.