First of all, doom IE for LIFE. I seriously hate it and I think it is deprecated.
BUT a lot of people still use it so here we go. Is there, in any sad way, a method to use IE6, IE7 and Firefox on the same machine. IE7/IE6 AND Firefox arn't a problem, but IE6 AND IE7 is not possible, for as far as I know. Installing a virtual server on the other hand is somewhat stupid.
I actually installed a seperate virtual machine to do this in the recent past, but yeah it is a pretty stupid approach to doing it I guess, now the easier method is using another computer I have with IE6 on it still instead..
BUT I know there are ways to emulate all the way down to ie4, I'm not quite sure of the name of what you're looking for, but I've given up a few times and just found it easier to use 2 seperate boxes, lol.. but i know for a fact there is a way to emulate older ie's.
Crap, my website doesn't seem to fully function in IE6 (according to the tool obolus provided) which is a shame. Downloading the IE6 from Rapidshare didn't work either. I couldn't initialize it since I am running Vista I guess. It immediately crashed.
Do not download the binaries for IE6 or IE7, I've experinced that both would appear just the same on code that should look different in IE6 and IE7 and therefore it cannot be trusted to do it like that.
Use the virtual PC images supplied by Micrsoft is the only "secure" way to do it =)