Very clever thing to do for now… of course, that’s just temporary solution…

Im using the built-in firewall of WinXP SP2. I'll see if I can configure it for avast.

Not necessary at all. You don’t have to touch Windows SP2 firewall, except if you wanna givie some permissions to some special software, like pcAnywhere, UltraVNC, messenger programs etc. …but, I’m sure you don’t have to change anything in SP2 firewall to give permission to avast! mail scanner…

My mobo does have a hardware firewall, but as far as I know it is not active. I can doublecheck this. I'm not sure about MTU settings. How can I configure those? On my old computer (Win98SE) I downloaded some utility to automatically apply good MTU settings. Should I try doing the same on my new machine with WinXP?

What’s your motherboard make, also if you can provide model name, it would be great as well… Are you sure hardware firewall is integrated into motherboard ?

MTU settings you can set if you go “inside” your firewall. Usually, accessing your firewall settings is through your web browser, unless you have some other type of router/firewall. See attached image… that is one of my router’s configuration pages… There are many web sites explaining how to set your ideal MTU value. Take a look in here, that may give you some ideas:

http://www.dslreports.com/faq/5793

Also, please take a look in here. Don’t be too lazy (I know you are not, but I just wanna be sure, hehe ;D), read everything that you find on that page. It could be that solution for your problem is somewhere on that page:

http://www.google.ca/search?q=cache:XQtQUcbgwJQJ:www.globetechnology.com/servlet/story/RTGAM.20030514.gtss1may14/BNStory/Troubleshooter/+mtu+mail+problems&hl=en

Cheers !