I've seen a few misconceptions.
One guy mentioned that he used the license on his laptop. Manufacturers actually have a volume license. Your laptop came pre-activated from the manufacturer using a volume license. This key is used on multiple laptops fromt he company to help accelerate the manufacturing process. Many manufacturers do this. So, that key on the bottom of your laptop is a fresh, never been used, key that's intended for re-installation purposes without the need of using the recovery partition on the hard drive (face it, if you get a bad virus that kills the recovery partition, you'll need a CD and a license). So, if something happens to that recovery partition, you'll need to get another key since the one on the bottom of the laptop is in use on another computer. No, you can't uninstall Windows off that computer and use the license on your laptop. It's an OEM key, so it's tied to that computer.
To the OP, Windows won't activate if you already used an OEM key on another computer.
I have never heard of this 6 month thing where MS only keeps a record for 6 months. It sounds like a myth from the internet. If MS was to do this, it would completely defeat the purpose of activation in the first place.
What the system basically uses for activation to determine if it's on another computer is the motherboard. Well, from various articles that I've read over the years, it's a point system. Certain components carry certain points. However, the motherboard will max out the points if changed and will fail activation. So, a different computer=different motherboard which means activation will fail.
You can reinstall the OS on the same computer as many times as you'd like.
If you buy an OEM copy, you don't need to worry about buying an upgrade version, since there is no OEM upgrade CD.
