If you use Virtual PC you really should read this
http://andrewconnell.com/blog/articles/UseVirtualPCsDifferencingDisksToYourAdvantage.aspx
Andrew Connell, fellow MVP, has this great blog on using differencing disks to drastically cut down on the amount of storage you need for VHDs. He also has some good tips on getting better performance and how to setup a base image. Check it out if you get a chance.
Shane