I've been using Outlook Express for several years now. I've never once passed a virus onto anyone.
I bet you have a good firewall, and have an awareness of computer related isssues, My wifes family have none, have trashed 3 machines due to viruses, but the issues vanished when I changed their mail clients, and installed firewalls, and other fixes, every month or two I update the machines, and since then viruses and worms have been a non issue, Even thought they open every attachment they get (don't ask me why), But if you have less than skilled users, do them a favour, move them off OE. Not off windows, just off OE.
On a side note, ever since I installed googlebar onto my family's PC and adjusting security levels ever so slightly, it has stopped ALL issues of programs trying to install themselves and any other sites taking over my parents homepage and such. Plus also got them a domain and bumped their email through that via my mail server which runs qmail-scanner-queue (CLAMAV and Mail::SpamAssassin) and haven't had a single problem since! FWIW