MacBookPro '09, OS 10.7.5, Sophos Version 8.0.20C
Click on Start Scan and "calculating" message remains for over 12 hours now. Second time I tried.
This thread was automatically locked due to age.
There is a recent (and unresolved) thread here on the same thing. Probably worth a read for the suggestions.
However one obvious difference is that you're not running 9.0.5. Not saying an upgrade would necessarily fix it, but version 9 is generally a better version to be running.