I see a couple of misconceptions that should be clarified:
1. The Outlook/Exchange provider does NOT handle Outlook Express. OE is a totally different program from Outlook, with totally different messaging model and plug-in model. In avast, OE is treated just like any other POP3/IMAP4/SMTP-based e-mail client (Eudora, Pegasus, IncrediMail, Mozilla etc.) and is covered by the 'Internet Mail' module. Hence changing any of the properties of the Outlook/Exchange shield in avast has absolutely no effect unless your using (full-blown) Outlook.
2. The "Scan Archived Messages" option in the Outlook/Exchange provider means whether or not scan messages that are not in transport (inbound, outbound) but rather are already stored in the Outlook folders (Inbox, Sent Items, Drafts etc). I agree, it'd be more appropriate if this option would be called 'Scan Stored Items' but that doesn't change the fact that the option does what I just said. SO, it doesn't have anything in common with archive scanning, i.e. scanning of "packed" files (ZIP, ARJ, RAR etc.). To set the options for archive scanning, you'd need avast! Professional Edition. There you could finely customize which archives avast should scan in which provider.
3. While thinking that archive scanning could have some connection with the problems you're experiencing is not a bad idea, I don't think it's correct. The reason is the following: the scanning itself (i.e. also the decompression in case of packed objects) is performed inside the avast main service process ashServ.exe, NOT in ashMaiSv.exe (the mail scanner). This is how it works. However, what you're seeing is a runtime error inside ashMaiSv.exe so it's quite likely that the core of the problem is elsewhere. My personal tip is the heuristics and/or PUSH module, because it's probably the part of ashMaiSv.exe that actually tries to dissect the message (i.e. is sensitivite to input data).
Hope this helps,
Vlk