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Windows 7's built-in virtualization tools let you do all kinds of useful things -- from multi-booting using virtual hard disks (VHDs), to mounting those same VHDs as local disks, to running older software that doesn't play well with OSes newer than XP. It would, however, be nice if certain tasks were a bit simpler.
Fortunately there are some great, free tools which can help.
Disk2vhd
http://technet.microsoft.com/en-us/sysinternals/ee656415.aspx
Need an easy way to convert a physical machine to a virtual one? Fire up Disk2vhd and create a snapshot of any system running Windows XP or later -- including server OSes and x64 builds. Partition structure is retained, and you can even save your VHD to the same drive you're backing up.