Your advice was sound and once I returned the TEMP directory to the C:\Users\User\Name\AppData\Local\Temp folder I was able to install Avast. The nice little lady even told me the Virus Database had been updated.
The only problem was that I want to reduce the number of writes to my SSD so I was not happy with the TEMP and TMP folders being on C:\. After a bit of trial and error I found the best way to do this and still be able to use AVAST was:
1. Use Link Shell Extension to move C:\Users\Raider\AppData\Local\Temp off the C Drive (see here for instructions)
2. Install AVAST! on a Hard Drive (say D)
3. Use Link Shell Extension to move C:\ProgramData\Alwil Software to a Hard Drive.
This cuts the AvastSvc.exe writes to C drive considerably. There are still a lot of 4.096 byte writes going to C:\Windows\System32\config\SYSTEM but I don't want to mess with these!