OEM Vista installs are motherboard dependent. As long as you don't change out your motherboard you should be fine with activation otherwise you'll probably have to contact Microsoft to have them reset your activation.

OEM licenses defer support to the system builder. You wouldn't have support from Microsoft would something happen. Retail licenses are more expensive, but grant you support from Microsoft and the activation limit is generally more lenient than an OEM install.

Another more questionable method is to find an OEM install of Vista running from a computer at work/school or maybe from a friend who doesn't use it and run a program called Activation Backup and Restore to extract the key from the OEM install, then use a Vista Anytime Upgrade DVD (which contains all versions of the OS, although there are two versions of the Anytime Upgrade DVD for 32-bit and 64-bit installs) to install a fresh, unactivated version of Vista. The same program will then restore the extracted key to your install and activate Vista with it.

I've used this method to installed a clean, non-bloatware copy of Vista on my dad's laptop no problem, although I'm not sure what would happen if the same method were attempted to restore a key that was already in use. It wouldn't hurt to try. Microsoft discontinued the Vista Anytime Upgrade DVD program because people like me were abusing it, but it's still relatively easy to find a copy on ebay.