How do you log on to your computer if you lost your password?

Another user of that computer could log in with his password and may be able to change your password. Administrator status is probably required.

Sometimes the repair or install CD that came with the computer (or was made when the computer was new) has a feature to clear the passwords. Careful, these disks can also restore the computer to original condition, deleting your files.

There is a password resetting Linux CD that you can boot from and blank your Windows password. It is possible to download the free CD image and create your own CD, but it is much easier to borrow one. The one I have is quite easy to use and works every time.