How Can I Deactivate Vista?

You don't deactivate it. If you would like to reinstall Vista on your existing system, just do so. Microsoft allows a few activations for the same product key on the same hardware. If this is an OEM version of Vista, your license says you can only install the product on a single system and then it is non-transferable. OEM licenses come with systems from manufacturers such as HP, Dell, etc. You can also purchase OEM licenses, but you would typically already know what you have. If you are getting rid of your computer and replacing it with another, just install Vista and activate it on the new system. You can wipe out the old system just by reinstalling Vista on it and not providing the key or by using any number of utilities to wipe out the partition on it.