Because if VMware & Parallels have software made to dump your existing drive and turn it into a VM. That would be the better choice. You have something that has been tested by those company's instead of a small utility.
Try it out, I can't promise it will work though. I dumped a old laptop drive using disk2vhd and ran it in VirtualBox, it worked but I would get random BugCheck's (BlueScreens) from time to time.