That´s a big problem my friend....
One method of gaining access to the system is by trying hard to remember the forgotten password, or a password of another user which has the same level of administrative rights. However I don't think this approach will help you, otherwise you wouldn't be sitting here reading this, would you?)
Another method is by trying to restore a backed up System State (in Windows 2000/XP/2003) or a ERD (in NT 4.0) in which you do remember the password. The problem with doing so is that you'll probably lose all of the recently add users and groups, and all the changed passwords for all of your users since the last backup was made.
A third method might be to install a parallel operating system on a different partition on the same computer, then use a simple trick to gain access to the old system.
Windows Password recovery
http://www.loginrecovery.com
This site provides a tool to recover lost Windows XP passwords. It works for administrator and user accounts, it doesn't change the password just tells you the old one. It works with encrypted files (EFS) and password hashes. It even works if no passwords at all are known for the machine (as long as you have another computer with internet access to view this website with).