Windows only: Free Sysinternals application Disk2vhd takes a snapshot of your PC, and turns it into a Microsoft Virtual PC virtual hard drive — while your system is up and running.
Using the software, which requires no installation, is as easy as launching, selecting the drives you would like to create into a virtual disk, giving the file a name and clicking the Create button. The utility uses the same volume snapshot feature that Windows uses for backup utilities, so it doesn't matter if software is running or not — but you'll have best results if you create the virtual disk on a separate drive.
Once you've created your virtual drive, you can use the free Microsoft Virtual PC to boot the image of your PC, which you can then use for testing, or just as a great way to completely backup your computer. Disk2vhd is a free download for Windows only.
Disk2vhd v1.0 [Windows Sysinternals]