You can run 2 MS clients at once in W7

You can't do it in vista without some sort of program (and most of them ban you afaik). With this you don't need anything, you can just switch users with you want to control a different character. Though i'm pretty sure you can do it in XP.

xp and windows 7 work this way but somewhere in the coding for vista they messed with how your computer handles concurrent user accounts so it doesn't work