A few things about virus protection on the Mac...

-don't turn on filesharing unless you need it and your behind a firewall (turned off by default)

There's a little bit of awareness needed for Microsoft documents opened in Microsoft applications. The macros built-in to these applications could cause some mischeif. Nothing too serious though.
- don't use a Microsoft email client
- don't open Microsoft documents emailed to you from people you don't know

-The software updater program is set up to look for new updates weekly. It will alert you if there are any. Be sure to download these regularly. (I usually wait a day or two after they have been released to make sure that all the kinks have been worked out before downloading. Any security updates will be distributed this way.