What is the best means of prevention against viruses?

A.
Assign read only permission to all files on your system.

B.
Remove any external devices such as floppy and USB connectors.

C.
Install a rootkit detection tool.

D.
Install and update anti-virus scanner.

Explanation:
Although virus scanners only can find already known viruses this is still the best defense, together with users that are informed about risks with the internet.