Use the antivirus software that comes with your computer.  Most computers sold after 1998 should come bundled with antivirus software, such as Norton Antivirus or McAfee Virus Scan.  If you don't have antivirus software or it's out of date, GriSoft  makes a free antivirus program.  Good antivirus software scans executable files before the are run, completely transparent to the user.  You'll never know it's there except for the icon in the System Tray. (Lower right corner of your screen by the clock)

Update your Operating System.  Microsoft is continually finding security "holes" which computer virii will exploit to get into your computer.  Use Windows Update to keep your computer relatively secure.

Turn off Outlook Express's Preview Pane.  The top 2nd computer virus (WORM_KLEZ.H according to Trend Micro's antivirus.com as of 9/26/2002) enters computers by running an executable script through that Preview Pane, and mails itself to every e-mail address it finds on the infected system.  With the preview pane open, merely highlighting an e-mail to delete it can cause a virus script to be run.

Don't open e-mail attachments.  Only open them if you specifically requested them or if you know the file extension and what it's capable of.  File extensions are the portion of the filename after the period.  Some Executable file extensions are *.exe, *.com, and *.vbs.  Anything with an executable extension may introduce a computer virus into your computer, whereas extensions *.bmp, *.jpg, and *.gif are harmless picture files.

Update your antivirus software.  Computer Virii change all the time the only way to stay ahead of them is to update periodically (bi-weekly or monthly is good).  An antivirus that was updated a month ago won't find a virus that was discovered yesterday.

Use your antivirus to do a complete scan of your hard drive periodically.

Use an external virus scan.  As many virii are designed to defeat the software that detects them.  Trend Micro offers a free web-based virus scan at http://housecall.antivirus.com, which I highly recommend.