I've been using Avast Home over 3 years, on XP Home, with Thunderbird and Firefox, with no problems whatsoever.
This forum provides some of the best free program support on the web, I reckon. I don't think there is any "live" support, so to speak, emails to the company etc would probably be it.
Often asking about any issues on the forum will resolve issues prior to needing to ask the company.
If you do switch to Avast, I'd suggest downloading the full setup file first, available
Here, (direct download, English version, 47.5 Mb), save it to the desktop, download the AVG remover from
this page, get the version appropriate to your OS, 32 or 64 bit, going offline (disconnect the computer from the web), uninstalling AVG via the control panel, reboot, run the AVG remover, reboot if prompted, then install Avast from the full setup file.