I have two virtual machines (MS Virtual PC) on my Vista laptop. On one of them, I use it to run MS Office and all my other XP applications.

On the other one, I use it for Internet browsing and general testing of new applications. When a deadly virus hits the virtual machine or I have installed/uninstalled too many applications, I simply delete the machine and create a new one. It's just about two hours of work versus a whole day to re-install all my applications plus the stress of possibly losing my data.