Make regular backups of critical data
Keep a copy of important files on removable media such as ZIP disks or recordable CD-ROM disks (CD-R or CD-RW disks). Use software backup tools if available, and store the backup disks somewhere away from the computer.

Make a boot disk in case your computer is damaged or compromised
To aid in recovering from a security breach or hard disk failure, create a boot disk on a floppy disk which will help when recovering a computer after such an event has occurred. Remember, however, you must create this disk before you have a security event.
For further Reading: http://www.us-cert.gov/reading_room/home-network-security/#IV-A-12

Recovering from a Virus
Sometimes using an anti-virus software package to "clean" the system may not be enough.  In general, the only way to ensure that a compromised computer is free from backdoors and intruder modifications is to re-install the operating system and install patches before connecting back to the network.
Source – US-CERT http://www.us-cert.gov/cas/alerts/SA04-079A.html

Also see: Understanding Viruses and Worms