@hazardwarning I'm in the same boat with a hugely bloated Vista on an Acer laptop. The size problem is down to the winsxs folder. See this link:
http://www.thewindowsclub.com/winsxs-folder-windows-7-8
You can clean it up a bit with VSP1CLN.EXE. Just bear in mind that if you have Vista Service Pack 2 installed they have replaced that with compcln as explained in this site:
https://bcrawfordjr.wordpress.com/2009/08/08/windows-vista-service-pack-2-clean-up-tool-compcln-exe/
I did the compcln command but it only got me 1GB back so I think re-installing Windows is the way to go. Once I've done a clean re-install and removed some of the Acer bloatware I may use @matigo's VM method.
// @matigo