[vkr]

If booting from a CD or USB is allowed, you can just change the password using the Pogostick [1] live cd - or any linux live cd.

[1] http://pogostick.net/~pnh/ntpasswd/

[dguido]

And then you can't set it back (since you don't know what it is), so the owner of the machine would immediately know you were there. They wouldn't be able to log in.

[StuffMaster]

Actually you can, by exporting/importing the right registry keys. Or at least you could on XP.

[xenophonf]

ntpasswd isn't a pentesting tool; it's a recovery tool.