Do not open unexpected email attachments, even if it is from someone you know.
Be suspicious of everything. Someone with whom you regularly correspond could be sending out viruses without even knowing it. Additionally, viruses can "spoof" the "TO" fields of an email, so it may appear to come from JohnDoe@xyz.com when it is actually from trouble@abc.net.

Create a GroupWise rule to filter viruses
Send emails with potentially infected attachments to another folder rather than your mailbox so you won’t accidentally open an infected email. For instructions on creating the rule: Virus Rule

Be extremely careful of what you install from the Internet.
Read those messages that automatically popup and ask you to download something - you don't always need the item that it asks you to install. You may receive a message that claim your system is already infected, and you need to click on the pop-up and run an anti-spyware program to remove it. Do not fall for this scam. It may look official but it is not. Online games, e-cards and screensavers can also contain programs to log your internet activities. Please refrain from downloading these items.

Do not download peer to peer file-sharing programs like Kazaa, Morpheus, or Limewire, which also open a door for others to enter your computer.

Be aware of potential hoaxes.
There are emails that are frequently passed around that tell people to delete suchandsuch file on their machine because it is a virus. People find the file and delete it, only to discover later that it is a valid system file! There are also hoaxes about lost children, companies tracking how many people receive a certain email, $200 cookie recipes, and various urban legends. Before taking action is the Computer Incident Advisory Capability: http://hoaxbusters.ciac.org