One of the best defenses is to ensure that your real-time antivirus scanner inspects all files being written to the local hard drive. If something malicious is being planted on your machine, then you want it found and stopped before it can do any damage.
Another good option to consider is a program which blocks malicious behavior. Some of these include Achilles Shield, Secure4U, Esafe and Finjan.
For more information on this topic, visit these other searchSecurity resources:
Best Web Links: Malware
David Strom's Security Tool Shed: myCIO.com Virus Scan ASAP
Scheier's Security Product Round Up: Antivirus tools unify and conquer
This was first published in February 2002