If you’re running IE7 under WinXP, in order to run Windows Explorer with the runas command, it must be run as a separate process. There’s a couple of ways to do this, but one that I recently found which is very slick is with the undocumented parameter /separate, like this:

runas /user:domain\username "explorer /separate"

Oh, and if you’ve got a space in your username, enter it as /user:”domain\user name”