Be sure to do a monthly full scan with your anti-virus client. (Get the updates first of course.)  Beware of any "free" tools promising to fix things in your computer as they tend to harbor malware and viruses.

Users of Windows XP, Vista or 7: be sure your anti-virus is up-to-date and that you have the latest Windows updates.  *** Limited Windows Vista support by MadMod Computing! ***  Use Ubuntu 10.04 or Windows 7 Home Premium (try to avoid Windows 7 Home Basic) on new computers for safer e-mail and web browsing if possible.  See the Ubuntu Diary for details.