If you're like most poeple in the Mac world, you probably use Microsoft Office (and if so, hopefully version 2004, and not version X). There are a few strains of a very old but very persistent virus whose names begin with "W97M" ("W97M.Thus.A" for example) out there that are still circulating, which propagate themselves through embedded macros and the "Normal" template. If you're using Office 2004, and think you've been infected, you take the following steps:

1. Quit MS Word (select 'Quit' from the 'Word' menu if it's running). Then go into your home folder and open the 'Microsoft User Data' folder. Take the file called 'Normal' and re-name it 'Normal-OLD' and move it to your desktop. If you have customized your "Normal" template, you will need to re-create your customizations after following these steps.
2. Launch MS Word, then go to the 'Word' menu, and select 'Preferences > Security'. Be sure the box is checked that says 'Warn before opening a file that contains Macros.'
3. Launch MS Excel, then go to the 'Excel' menu, and select 'Preferences > Security.' Be sure the box is checked that says 'Warn before opening a file that contains Macros.'

If you ever try to open a document, and you get a prompt warning of a macro, be sure to select the 'Do not open' option (and not the 'Enable Macros' option). Then set the file aside, mark it as infected, and don't open it. Let the person know who sent it to you that they sent you an infected file. Of course, the only exception to this would be if you're specifically expecting to receive a file with macros in it, in which case you can open a file when you see this warning.

Even having taken the steps above, it's a very good idea to install a good anti-virus package on your computer, keep it current, and scan your computer regularly. I personally have found Norton and Symantec products for Mac to be very problematic, and would not recommend using them. I prefer to use ClamXAV, and if you do use it, please make a donation to support the author of this program.