Ok, I don't wanna know what you do with the system, but if you have physical access to the system, you should not use knoppix/l0pht. This can take forever. First of all, a default install of Windows XP has a blank Administrator password, so if try to log in as Administrator first. Second, the tool to use is http://home.eunet.no/~pnordahl/ntpasswd/ which will allow you to reset any password you like.

Oh yeah, l0pht crack was bought by @stake, which was bought by symantec. They have nothing to do with Winternals/sysinternals.