Occassion1: Your computer is a Domain Server or Controller

In Windows Server 2000/2003(R2)/2008(R2), if the machine is a Domain Controller, other users except for the domain administrator can logon to system. This is because local admin password doesn’t have the authorization to login in Domain Controller for interactive logon is not allowed in the local policy. So, under this circumstance, you actually do not have a “real” local administrator. Similarly, you cannot reset local admin password. Just forget it, because you can still use the domain admin to access to your machine.

Occassion2: Your computer is a Domain User

When your computer with Windows Server 2000/2003(R2)/2008(R2) is a member of a special domain, not the domain administrator, you can access to your PC with 2 kinds of logon methods—domain administrator logging and local admin or standard user logging. So, in this time, if you forgot local admin password, you can do the local administrator password reset as follows:

• Step1. Find a credible password reset tool through the Internet.
• Step2. Here I trust Windows Password Recovery Ultimate program and decide to use it. So first download, install, and run it into an accessible computer.
• Step3. Insert a blank CD/DVD/USB Flash Driver into that computer. (Note: not the locked one.)
• Step4. In the interface of Windows Password Recovery Ultimate program, click “Burn” icon to burn the program ISO image file into that blank device.
• Step5. Insert the newly burned CD/DVD/USB Flash Driver into the target computer, and set computer boot from CD/DVD/ or USB Flash Driver. Next, reboot the computer.
• Step6. Once the computer is restarted, an interface will show up. You can see two kinds of Windows Path: C:\WINDOWS and D:\WINDOWS listed there. You just choose the former one to change local administrator password, while the latter one is for domain admin password. Next click “Reset Your Password” → “Next”. Now find your local admin account, click it, and then click “Next”. After finished, click “Reboot” to end all the whole process. Now you can use the newly changed local admin password to logon to your PC.

Occassion3: Your computer is neither a Domain Controller nor a Domain User

Computer being locked by forgetting Windows local admin password is the most common problem when using Windows XP/7/Vista/2000/2003/2008. But here, I assure you that it is not a big deal at all, you can easily find or change your forgotten local admin password. As for the easiest solution on how to change local admin password is what I have introduced in Occasion 2, please follow those step-by-step instructions when you are in need.

Source: http://www.windowspasswordrecovery.net/article/change-local-admin-password.html