The thing being is that you define your PC as a home PC, but you run your business on it, therefore using Grisoft's definition (
http://free.grisoft.com/doc/28394/us/frt/0), you have commercial use and therefore need a commercial version. Your phone provider may not apply the same rules (Presumably when you use your phone for business you recover this cost against business expenses showing the real cost of the business?).
In the case of software it's the vendor that defines the license not the user.
This is not a pop at you, but...
The full version of AVG is so cheap (retail for a single PC is just over 50p/week for the 2 year license) and offers more features than the free version, it surprises me how many people would rather violate the license agreement on the free edition than spend a few tax deductable £'s of their business money. There was major outcry a while back when Grisoft put in some code to detect whether the PC was standalone or in a domain and point out the licence violation to the user!