Fix for 2755 and 1632 ‘The Temp folder is on a drive that is full or is inaccessible’ errors

Whilst trying to install an MSI package we kept getting the errors 2755 and 1632 ‘The Temp folder is on a drive that is full or is inaccessible’.

After much fiddling we found the problem was that the %systemroot%\installer folder was missing. Once this was manually re-added the MSIs installed without a problem. The actual TEMP folder setting was a red herring