On all versions of Windows, you must ensure LetSys (and the Configuration program) are not running before you upgrade to a new version. Here's the detail of this problem:
One of our users reported that they were unable to upgrade the programme when running in Vista.
The message was:
C:\Program Files\LetSys\LetSys.exe
An error occurred while trying to replace the existing file:
DeleteFile failed; code 5.
Access is denied.
When you abort the installation (the only sensible option in response) then the system is "rolled back" and everything still runs at the previous level.
The reason for this was simply that LetSys.exe is in use as the program was running. Shut it down and try again.
Brendan Blake.
Failed to install in Vista "Deletefile failed; code 5"
Moderator:bblake