Nearly every virus out there targets Outlook so users of any other mail client are pretty safe. As long as your client doesn't automatically open attachments with the message there's not too much risk. I use Netscape at home, Notes at work and have never had a virus problem.
Windows itself isn't that bad if you keep it constantly up-to-date (the same can be said of Unix, but most home users aren't aware of the number of patches that get put out for Unix security problems). But Outlook? That thing is just a virus with scheduling capability. I mean, it's just one big security hole.