[reboot]

Pegasus or Eudora, either lite or Pro.
Simply turn off "Use MS Viewer", and it's done.
Your computer will no longer automatically run any attachment, even if VBS is installed, and it won't automatically send anything either, because it uses a completely different type of address book, that the trojan cannot read.
It's not the OS, it's Outhouse Express. OE (no matter what version) attempts to do something with every attachment. If it's got the right sort of suffix, you're infected. Period. Unless you update and run the virus scanner, and have email filtering on before you download any email every day.
Even then, you can get infected, if the trojan/virus is new enough to NOT be included in the latest A-V definition.
Once infected, (most) trojan payloads read the address book, then propogate by sending themselves to everyone in the book, and at the same time spoofing the senders address as someone else in the address book.
Pegasus and Eudora don't allow this at all, and all attachments show up as attachments, in a folder, simply deleted.
Eudora light is probably best for single user, Pegasus next, although it's much better for LAN/WAN as well as POP mail. Eudora pro, even with it's free version ads, is still a superb email client.
Not sure about Pegasus, but in Eudora, you can even turn off any html stuff, and send/recieve email in plain ascii text, so even if someone sends you an email with "stationary" (fancy background/fonts/colors) it will show up as plain text, and all the extras will show up as attachments.
Become part of the solution instead of the problem, and uninstall OE.
For those of you who use OE for newsgroups (usenet), just get a 3rd party reader. There are a number of good free ones, well worth the download.