Service Pack 2 will not allow installation on a system which currently has a beta or release candidate version installed
The final Service Pack 2 will update some beta versions, such as those made available through public preview programs. However if you are trying to install the final SP2 onto a system which currently does not have SP2 build 2096 or 2138-2179 installation will be blocked. If this is the case you are trying to perform an unsupported upgrade and have 2 options. First to uninstall the beta build of SP2 currently installed.
The second is to get around the setup block. It is possible, but it is blocked for a reason! Your system may not work correctly or beta components may be left on the system. Do this at your own risk.
1) Go to Start > Run and type Regedit
2) Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion