The windows service is "Started". (I have been checking that it was started whenever I get the error, it usually is although on the one or 2 occaisions I have seen it stopped.)
I've set recovery options but still get the same error. (Recovery just set to restart service)
Tried a few restarts too with no joy.
To be honest I was assuming no difference, recovery I assume is just about when the service crashes, and that's not what's happening here..
From scm it looks like the service is running fine, I can start/restart etc.. with no errors... it's just that whenever i run any msi it says "The Windows Installer Service could not be accessed"
BTW: Thanks for all your suggestions and help so far...