For various business reasons, my dad must leave his medical billing company and use a competitor, which is far less intuitive and more expensive. The entire system is application-based, and all computers must run either XP Pro or Vista Business. One of his computers at the front desk has Vista Home Premium. We have an extra copy of an XP Pro Upgrade sitting around. Can I use the XP Pro Upgrade to retrofit on the computer that currently has Vista Home Premium? Thanks.