It is unreasonable to offer, as a "solution", something that requires that a potential Avast customer go research this issue in the forums for a "workaround" requiring them to trust that whomever is providing the instructions to "press TAB twice, left-arrow once, down-arrow three times, hop on one foot and sacrifice a chicken on a Tuesday after a full moon" will actually work and do no harm is simply offering unicorns and rainbows for what is being passed off as "technical support".
You know what the real workaround will be? Uninstall this junk software and install something that ... works. The first time.
This is not something that needs a workaround. It is something that the UI designers that ignored physical constraints like a 800x600 maximum phyisical screen resolution, which is very common in netbooks and older tablets, should be held accountable for by misunderstanding the word "requirement", and entered as a DEFECT in the product to be corrected and fixed.
It should be reported to Avast Technical Support (I did) and I won't use Avast products or recommend same until it is. In the software development industry, it's called a critical block bug/defect -- when a customer cannot install your software due to your UI design *choices*.
In the end if Avast doesn't want to support such 1024x600 or 800x600 screen resolutions, then the installer should detect this and explain to the user that the installation is being terminated for this reason. But they didn't, so the UI requirement to support this resolution should stand, and Avast should fix the defect.