Just because I work for EMC does not mean that I have to use VMware over Virtual PC or any other Virtual OS software. I just find VMware very easy to use.
I have been using VMware for about 4 years now for all of my branding projects. I store them on an external drive and back it up frequently. This allows me to take all of my work on the road or to any client site as needed. As you can see from the screenshot on the left I normally start all of my projects from a base vanilla image of SharePoint. I have blurred out some of the names to protect my clients names.
On this image I have all of the tools I need to create my branding masterpieces. (SharePoint Designer, Office 2007, IE Developer Toolbar, and Visual Studio)
The great thing about VMware is that I can always go back to any of my past projects and make edits or changes as needed. Say for example you finished up a project about a month ago and you are knee deep into another project. When all of a sudden you get an email that there was this weird bug that needs to be fixed. All you would have to do is save a snapshot of your current project and open up the snapshot of the old project make the fix and you are done.
I guess the trick is to remember to save those snapshots. They really don't take up that much room on your drive.
If you have not tried out VMware you can try an evaluation copy here: http://www.vmware.com/products/ws/