How can I log into my computer if I don't know the password?

If you computer is a Windows PC, restart it, then hold the F8 button on your keyboard till a screen comes up asking you to choose a startup mode. (If this screen does not come up, and your computer arrives at the login screen again, then try pressing F10.) Choose Safe Mode from this menu, (with Networking if you want to be online,) then wait till the regular login screen appears again. Choose the user account Administrator, then you are now logged in.

By the way, while you are in the administrator's account, it is advisable to
  • A. Make a password reset disk the User Accounts section of the control panel
  • and B. Set a password for the Administrator account
These two things should both keep your computer safer from attacks and you logged in.