Heh, I use two virtual machines. One with Win98 and IE6 and one with WinXP and IE6.
The reason is the IE6 on Win98 at it's latest update is just slightly behind the one on WinXP, which you'd think isn't that big of an issue but for some reason things don't work in both all the time.
Outside of that, I generally also use a VM with FF2 and IE7, on top of IE8, FF3, Opera, Google, and Safari on my host machine.