"KB2600640 is not installed on this computer." ,

Hi, we have only Rollup 12 on our server crm 2011 (not having rollup 6), we installed 12 directly there was no problem while installation. Now we want to install rollup 18 but it gives error

"KB2600640 is not installed on this computer." ,

when we try to install rollup 6 its gives error

"---------------------------
Microsoft Dynamics CRM Update Rollup 6
---------------------------

The upgrade cannot be installed by the Windows Installer service because the program to be upgraded may be missing, or the upgrade may update a different version of the program. Verify that the program to be upgraded exists on your computer and that you have the correct upgrade.

later we tried to uninstall rollup 12 but we cant as same error :"KB2600640 is not installed on this computer." ,

Is anybody have solution for same?