Mainly use it for testing Linux distros on. For windows 95 through XP you could also choose to try Microsoft's VPC2007. I have windows 98SE on a VHD in Virtual PC 2004 under my current XP system. Virtual drives are great because you can transfer and use them on any PC that will run the virtual software.
I can see a future time when the free operating systems will all come as a VDI or VHD and motherboard manufacturers will ship their motherboards with virtual software already as firmware and you can import the operating system file of your choice.
May fix a lot of headaches. Including mine.