There are as many utilities for bypassing Windows (and Linux) login passwords as there are legitimate reasons for doing so (you ARE only using your powers for good, right?).

Kon-Boot can help you to bypass a login password. It can be run from a boot CD, floppy, and (thanks to Irongeek), from a USB flash drive.

Need to help a friend (or yourself) to overcome a password-protected Windows or Linux computer? Check out Kon-Boot.