Donít Rebuild Virtual PCís, Copy Them
At work, I run Vista Business as my primary OS and I use several virtual PCís (my tool of choice is Microsoft Virtual PC 2007) with Windows XP loaded on them so I can test development projects and have the ability to maintain a common ďalternateĒ environment. For obvious reasons, this is preferred over having a different physical machine or a separate boot for each environment.
One very handy trick I found is that you can copy virtual hard drives (.vhd) files and instantly get another environment. For example, you could build a new virtual PC hard drive and load XP with all the updates and then simply copy the VHD file to a new file name and, whola!, you now have two ready to go environments. All you have to do is just run through the virtual PC creation wizard and set it to use an existing hard drive (your copied VHD file) and you are ready to go.
If you have to deploy several environments quickly, this is a great trick to know.