I've installed several instances of Windows 2008 and Windows 7 in my VirtualBox and they are really huge. I've read something from the Net and it seems the WinSXS folder consumes the most spaces. If I understand correctly, this folder saved all versions of Windows components and it's an increase-only folder (i.e. no remove, no modify). Therefore, here is my suggestion:
Is it possible to let all Windows guests sharing the same WinSXS folder? The names of folders inside it include the architecture name (say, x86, wow64, or amd64), so this means it can be shared between Windows 2008 and Windows 7. Now, if your host is also a Windows, it can shared between host and guests. This would save a huge space.
I'm not sure how Windows Update works. Hopefully if a component is already stored in the shared WinSXS folder, the update process needn't download it at all. Also, I don't know if there will be some problem when two systems are accessing this folder at the same time. Hopefully this won't be a big problem, because I see no meta info installed this folder, it's likely that each system manages its own which-components-get-installed info in its own database.
Thanks
Cary