Is there a way (when logged in as an administrator, or as a member of the administrators group) to masquerade as a non-privileged user? Especially in an AD environment.

e.g., in the Unix world I could do the following (as root):

# whoami
root
# su johnsmith
johnsmith> whoami
johnsmith
johnsmith> exit
# exit

I need to test/configure something on a user's account, and I don't want to have to know their password or have to reset it.

Edit:
runas won't cut it. Ideally, my whole desktop would become the user's, etc. and not just in a cmd window.