Running an entirely Linux household (wheezing old eMac notwithstanding) works really well for us. Dell go through phases of being utterly in love with Linux: to the point of shipping machines with Ubuntu preinstalled; in between those phases, they at least do some grudging certification, so you know the hardware will more or less cope.
Sometimes, though, circumstances require a non-Linux environment. The most frequent use case is for testing a website in (various different versions of) Internet Explorer. This used to be a nightmare for web developers, although recently Microsoft very grudgingly released some VMs specifically for testing IE (they won't let me permalink to the Linux VMs themselves.) I'm not sure what eventually prompted this: I imagine that the the "IE6 must die" campaign didn't succumb to Microsoft's attempts to massage and contain it; the campaign's attention was probably starting to focus on the sheer hassle of supporting even IE7; given how difficult Microsoft had been making such support, who knows where that developer fervour might have led, once unleashed?
Anyway, there are these VMs available now. But you'll be astonished to hear that something provided by Microsoft looks like its download method was invented in a parallel universe: spread over several RAR files, using self-extracting archives etc.... and by the end of it, you're kind of on your own when it comes to getting them running in VirtualBox. It's almost as though Redmond don't know yet that Vagrant's straightforward management of .box files is even a thing; or if it is a thing, it's certainly not invented there.
Enter xdissent's IE VMs script. This is a tool freely available on github for a single-command install of VMs for all current flavours of IE (6-10). And it works really well: once I'd increased my machine's memory substantially, I got the script to download and install just the IE9 machine as a test, and before I knew it, VirtualBox was happily running a slightly lobotomized version of Windows 7 for me. Frankly, after I'd had a purely Linux machine for so long, the sight of it running another OS took some time to get used to.
Speaking of lobotomization, the IE VMs are great for that common use case, but Microsoft have obviously locked them down to prevent them being too useful outside of IE testing. But there are still non-IE tasks you can perform with them: another use case is that of third-party software: recently I missed out on a webinar hosted on the execrably Linux-incompatible GoToMeeting, which I'm told will more or less install on an IE VM. I daren't ask whether or not iTunes will run on one; right now I'm too busy luxuriating in this modest yet unequivocal success.