Virus Protection.
Virus protection is essential & makes up the first of the "Holy Trinity".
It is important to get virus protection that includes some kind of "resident shield" to protect you all of the time, not just when running a scan. For this reason it is vital you have only one piece of virus protection software at any one time.
Virus protection is only as good as its last update. Now-a-days most software comes with an automatic update option, but you must ensure this is working correctly & not being blocked by other software (eg. a firewall).
There are lots of software available, both commercial & free. It's best to choose one that you are comfortable & happy using. Bear in mind that most free virus protection software comes with some limitations, depending on the supplier.
avast! 
Description:
Virus protection that includes resident protection for a number of items & the option of scanning either your hard drive or removable media. Some advanced features including a skinnable interface & the ability to have it E-mail you if it detects a problem. The free version lacks the ability to schedule regular scans, but does allow for automatic scanning when the screen saver is active.
PROS:
Good interface.
Some ingenious features.
Resident protection.
Automatic updates.
CONS:
Not fully compatible with Zone Alarm (but does work around this).
May need manual configuration if you use unusual E-mail ports.
Does not scan SSL E-mail (but neither do most virus software).
Cannot schedule scans with free version.
Price: Free for non-commercial, home use.
Verdict: I've been using this software as my virus protection for a while now. One of the best I've found.
AVG Free
Description:
Grisoft was one of the first companies to offer free virus protection. They are still thought of as one of the best free providers.
PROS:
Resident protection.
On demand scanning of hard drive & removable media.
Automatic updates (limited to one per day on free version).
CONS:
E-mail scan keeps forgetting settings.
Interface could use some work.
Does not scan SSL E-mail accounts.
Price: Free for non-commercial, home use.
Verdict: Solid all-round virus protection. Very popular.