I got the same error popping up (error 80040605) when I opened email messages.

I have WinXP Pro, and it started happening in Outlook after I successfully upgraded Office 2003 to Office 2007. I have Avast 4.7.652 running in Netclient mode in my domain.

I was able to fix it by running the netclient install again, uninstalling the Outlook/Exchange feature, restart, than running the netclient install again and putting the Outlook/Exchange feature back in.

After that, it worked fine for me. Maybe if you did the same thing with Home/Pro, it would solve it. (I thought I’d post this for continuity’s sake…)