If your 64-bit Windows PC generates this error – “Windows Installer Service cannot be accessed and may not be installed correctly” when trying to install some programs, you should first read this Microsoft article. Following the steps in this article is a good way to fix any real underlying problem with the installer service.
But, if you are running a 64-bit version of Windows, you may have a simple registry problem that is preventing the proper use of the installer service even though the installer service is porperly installed. In that case, a simple registry change may be the last remaining step you need to take to get back on track. Note: the following steps involve editing the registry on your PC. If you make a mistake while editing the registry, your machine may no longer boot properly, so don’t try this unless you have backed up your PC!

1. close all running programs
2. Bring up the “run” prompt by holding down the “Windows” key and the letter “R” at the same time.
3. Go to HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/MSIServer and right-click on WOW64
4. select “Modify” and change the Value data from “1″ to “0″
5. close regedit and re-boot the computer

Now try and re-install the programs you were having trouble with before.