Bypassing an administrator password is difficult, but not impossible. There are a couple of routes you can use to get around a forgotten or unknown administrator password, granting you access to the computer without the need to reinstall Windows. Depending on your computer setup, you may not have access to each option; however, they are both viable options for bypassing an administrator password.
Reset the password from another account on the computer that has administrator privileges. Log on to the other account while in safe mode, which you can access by repeatedly pressing on the "F8" key on the keyboard as the computer boots up. Once in safe mode, navigate to the "Start" menu, and then click on the "Run" button.
Type the phrase "control userpasswords2" (without the quotation marks) in the text box that appears, and then press on the "Enter" key. Select the account that you wish to bypass from the list that appears, and select the option to "Reset Password." This removes the password from that account, granting you access.
Download and run a free password cracking program like RainbowCrack or Login Recovery (see the Resource section below for both programs). Both of these programs can retrieve or reset an administrator password, allowing you to bypass it, and "crack" the system.