Old post at http://forum.avast.com/index.php?topic=41348.0 so I thought I would start anew. I had the same issue. Palm treo 750, Active Sync 4.5, Outlook Avast, Windows XP SP3. I know by a new phone and OS…I am cheap sorry and the palm has been pretty bulletproof.
Anyways, I left AVG went to Avast and my phone was having all kinds of buggy sync responses. Did a hard reset on phone and reinstalled outlook. Then of course before reading the above post I shutoff Avast and active sync worked great after multiple unplugs and reboots. Then I remembered seeing a splash screen for an avast outlook plugin. I went into outlook tools/options/other/advanced options/add in manager and unchecked the box for the avast add in. The I restarted avast full shields and checked active sync to find that it was all working. I use inet email anyway so I see no need for the add in. I use outlook for contacts, calendar, tasks, etc.
Hope this helps this dated subject as I know most have androids, iphones, balckberry, etc