Don't mention "reasonable quality of support from an anti-virus vendor" instead identify them by name and repeat it often!
I don't know if your credi card or local provides it, but here in the USA, you can dispute quality of product if it is purchased in your state or within 50 miles of home. Your home computer is within 50 miles of home.
But, when disputing the charge make it simple, "when I install it, it breaks my computer." When I remove it, it works perfectly."