Error message "The Windows Installer Service could not be accessed".

One day, I tried to install Adobe Acrobat 8.0 but failed. I got error message” Windows installer service could not be accessed. This can occur if the Windows Installer is not correctly installed. Contact your support personnel for assistance.”. I also tried install ITunes but get the same error.
 

I changed Window installer service to active and automatic, and use command “msiexec /regserver”, still no go.

I heard another possible root cause is the presence of wow64 with data set as 1 under the key HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/services/msiserver, change value back to 0 may fix it. But still no go.

Finally, I used the below solution on my Win7, the issue fixed, hope it's helpful for you.
Steps:
1.    Start Regedit
2.    Delete HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/services/msiserver
3.    Restart computer
4.    Copy this msiserver key from other (Vista or Win7)machine which works fine.
5.    Re-install applications, issue resolved.