OK, found a solution so posting it for everyone else
install avast internet security 30 day trial version
reboot
allow avast internet security 30 day trial version to uninstall its firewall (will do this with no user input)
reboot
it now has downgraded to avast free antivirus
register
now add firewall trial
let it install
reboot
this gives you TWENTY days trial of the firewall (well thanks)
cant say this whole thing gives me any confidence to buy, what with the lousy customer service and automatic uninstallations…nonetheless i know the antivirus is good from past experience so maybe i will get over this annoyance