Sometimes (e.g. when inspecting malware infested machines) it is very useful to be able to log in as a domain user without knowing the user's password with the computer off-line. How to do it? In every case, a local computer administrator account is available.
One option I am aware of involves dumping the cached domain credentials and then cracking them, but that means that password is no longer secret and sometimes is simply not practicable. In linux/unix, I would just do sudo -u 'johndoe' and be done. What's the equivalent in Windows XP/Vista/7?
Edit: just to provide context. This is for inspecting malware infected machines. Some malware will only start if you log on as the originally infected user. I can't put it back on the network to reset the password as it would be too risky. So, the box is off-line and I have a local admin account which is created at install time for troubleshooting.