Virtual PC 2004 on MSDN Subscriber Downloads
Microsoft Virtual PC 2004 can be downloaded from the MSDN Subscriber Downloads. For those of you unfamiliar with VPC; Microsoft bought the technology from Connectix that allows you to host several virtual PC's on a single machine. Each virtual PC lives in its own isolated envirionment, and is physically only a single file of the host machine. To communicate with your Virtual PC's you can use either shared folders or (virtal) network connections. You can even access your Virtual PC's from anywhere in your network! This piece of software is great for software developers; you always have a clean machine to try something out. I keep a freshly installed Windows XP on my HD, and ever time I need to try something I copy that image. No more screwing up of your host machine.
Another nice feature is that you can rollback any changes (if you want so ofcourse). Imagine having a development server and staging server that allow you to try stuff out, without having to worry to something wrong; you can allways rollback to it's previous state.
If you want to use Virtual PC, make sure you have plenty of memory, since the memory each Virtual PC uses is not virtual, but physical. :-)