Computer Account:                                                                    

A Computer account in Active Directory is very similar to a user account in Active Directory .Generally a computer account and user account are made from the same attributes . Like user account a computer account has a password but in Computer account the password is randomly generated . This password is supplied to the domain when the computer starts up which allows  a secure connection to be created between the computer and the domain controller . This password is automatically changed after 30 days . If the computer has not connected to the domain mpore than 30 days the computer still be able to access to the domain . The password for the computer account will be changed the next time the computer connects to the domain .

Resting the computer account:

Sometimes the password used on the local computer and stored in the domain for the computer account become out of sync. When this occurs  , you will get a message "The trust relationship between this workstation and the primary domain field " . When this occurs , the comoputer will need to re-added to the domain.

To perform administration on computer accounts inside Active Directory, open Active Directory Users and Computers from administrative tools under the start menu.

If you select a computer account, you can access the properties of the computer account by right clicking and selecting properties. The properties tab contains information about the computer like what type of computer it is, for example, a “workstation or server” or a Domain Controller with or without it being configured as a global catalog server.

To create a pre-stage computer account, open Active Directory User and Computers. Inside Active Directory User accounts, navigate to the OU that you want to create the computer account in. In the new computer dialog you can also set a user account that will be allowed to add the computer to the domain.

To add a computer to the domain, open Windows Explorer and right click on computer and select properties. From the system properties, select the option change settings and then press the button change. This will allow you to remove or add the computer to a domain.

To reset the password on a computer account, right click the computer account and select reset account. The computer will need to be removed from the domain and re-added again. When you remove the computer from the domain and place it in a work group, you do not need to reboot the computer before adding it to the domain again. Once it is added to the domain, you will need to reboot the computer to complete the process.

Reference :http://technet.microsoft.com/en-us/library/cc759279(v=ws.10).aspx  and http://support.microsoft.com/kb/216393