Thats weird I was using a modified 5.6 up untill the end of Jan 06 and my avg antivirus, adware , windows updates worked with no problems.
What network protocols do you have installed ? What OS ?
I have ipx,tcp/ip and netbui and none of the machines behind my sme server connect with no problems. Well actually I just remembered one problem I have had going back to version 4. I have is that I have never been able to receive files via Instant Messenger (aol,msn,icq etc). I can send but if someone attempts to send me a file it times out.