If you forget your password, there's an easy way to change it, assuming you have Administrative privileges on the computer running SQL Server. What you can do is login to SQL Server Management Studio using Windows Authentication, and from there you can change your password to something you can remember.
If your Windows user account is not a member of the Administrators group, or if you're running Vista with UAC turned on, you may want to make sure you're starting the program as an administator by right-clicking on it and choose "Run As Administrator". On XP, you'd hold down the Shift key while you right-click and you'd choose "Run As".
When you login to SQL Server Management Studio, be sure to use Windows Authentication.
In SQL Server Management Studio, find your user account under the "Login" node.
Finally, right-click on your account, choose "Properties" and change the password.