rlm,
thanks for starting a new thread.
My guess is that it must be a collision with another program running on your computer. It doesn't even have to be a 'program', it can just be a hardware driver or something... It's quite difficult to say, but I can asure you that WinXP is now in fact our premium and most-supported platform and there are literally thousands of WinXP users who are using avast! without any problems.
Could you please try to describe what software and/or hardware you have installed? I'm particulary interested in the "low-level stuff", if you know what I mean. Also, If you had a program that could list all drivers loaded in the system, and you could deliver this list to me, that could be of great help, too. I understand that these 'experiments' are quite time-consuming, but there's a not-so-small chance that we'll be able to finally identify the conflicting piece of code, and you'd thus help the whole avast community.
If you're not comfortable with sending this (kinda sensitive) information to this public forum, please feel free to send it to me as an IM or via e-mail.
Thanks a lot for you patience,
Vlk
BTW: Yes, it is possible to configure any e-mail client that's based on POP3 or IMAP4 to have it's traffic scanned by avast (provided you don't use SSL).