How to virtualize a physical computer ?
While you can find a lot of web pages and blogs asserting that you need to change some esoteric things (Windows registry and so on) in the running system before running it virtualized, fact is that it is not true :
You just need to clone the harddrive(s) in a file. End of story.
Basic disk utilities can do that (under Linux, BSD and MacOSX one can use dd), many backup tools too. Later on, you will convert the file to the desired virtualization format, which is trivial to do (tools depends on your virtual hypervisor engine).
Things are a bit more complicated if your system use several disks or partitions. But once again you just need to clone them. Several small utilities can do that for you, directly converting disks images in one file only. Under Windows, Microsoft provide the small and freely available Disk2vhd (go to the Windows Sysinternals page and download the utility) or you may prefer an automated tool like the free AOMEI Backupper. Again, many free tools exists, P2V is trivial.