Until recently, there were these images provided directly by microsoft for free(!)
They are VPC (virtual PC) images, but ran just fine (sometimes with minor tinkering) under virtualbox -- the only catch was that they are time-limited, so expire every few months (no big deal). The community rejoiced, and thought 'hey, maybe MSFT isn't so bad after all'.
Unfortunately at some point, they changed it so that the images would only work in VirtualPC (which is windows only). Many people (myself included) are upset. It's not so much the cost of not having to shell out for a copy of windows, but it's so stupidly convenient to just download their images... especially if you are unfortunate enough to need to test under IE 6, 7 and 7.... (just grab an image for each).
There was a friendly and helpful MSFT employee who chimed in on the issue on the virtualbox forums and looked into it a bit, but so far not much has come of it: http://blogs.msdn.com/petel/archive/...vpc-hosts.aspx