I give a lot of demos using a lot of our technologies, and I don't want to try to configure my laptop to run everything under the sun.  I leverage VirtualPC a lot, but the virtual harddrive files can get pretty huge.  The solution?  Get an external USB 2.0 harddrive and run the VPC from the harddrive.  If your laptop only has USB 1.1, you won't be able to run VPCs from the external drive, so you can purchase a USB 2.0 PCMCIA card. 

Instead of trying to move VHD files on and off my machine, I have a Maxtor external USB drive and a USB 2.0 card.  Just make sure to use convert.exe to enable file sizes greater than 4 GB.  Now, I can run Team System, BizTalk, Commerce Server, and lots of WSE demos using multiple VPC windows (client and an Active Directory instance).  This also saves me from having to continually move huge files on and off my machine (causing fragmentation of your drive).