Yeah - that's a good question. I had Vista Ultimate until this last week when my HDD started crashing. While waiting for my replacement HDD (warranty) I ran Ubuntu as my main OS and WinXP (in VMWare). The ended up being a very nice setup.

Now armed with my replacement HDD, I decided that Vista isn't really worth all the driver trouble and hassle (the OS feels inefficient in many ways and some other small things that are screwy have been building on my nerves).

I reinstalled XP (mainly because I didn't want to run iTunes in a VM and I can't get dual monitors to be as easily configured as I can in Windows!), and think that the only reason to run a Windows-based VM including Vista would be to test SQL Server, Exchange, or some other non-OS product from Microsoft where by installing it on your core OS would just mess things up way too much.

I'm tired of reinstalling my OS after loading a bunch of apps to demo software/hardware with and believe that a VM container is much better at doing that now.