Virtual PC 2004 Differencing Disks
Stop recreating that perfect VPC image over and over and start using Differencing Disks! If you use Virtual PC, chances are you have spent a lot of time building out an image that represents your desired end goal. But its a shame to 'dirty' that image and then have to start over with a new one the next time you need a clean image. If you use the Differencing Disk feature in Virtual PC, you will never have to build your base image over again (unless of course you want a entirely different OS). Using a differencing disk allows you to work with/change your use of a VPC image without affecting your core OS, applications, and settings.
Check out this tutorial on differencing w/ VPC on Microsoft TechNet.