You can purchase a Vista license at http://www.amazon.com/software when you
receive the license with the product key, click Start > right click Computer
Trial as in one of the prerelease (beta or RC) versions? Or trial as in
installed the release (RTM) version without a product key?
The former can't be done without reinstalling with the RTM version. The
latter just requires that you just purchase and input the appropriate
product key. There are many vendors available for this.