At a very minimum for computers that are connected to the internet, Wisper recommends virus protection to be running and up-to-date.  
These programs include free or paid versions of:

AVG
Avast
Sophos
McAfee
Norton
Bit Defender
Bulldog
F-Secure
Kaspersky
Nod32
Trend Micro
Symantec
Windows Live OneCare
ZoneAlarm
Microsoft Security Essentials

If you do not currently have one of the above products or another anti-virus program running, we recommend downloading and intstalling either Microsoft Security Essentials or AVG Anti-Virus
Security Essentials
http://www.microsoft.com/Security_Essentials/
AVG Free
http://free.grisoft.com/doc/download-free-anti-virus/us/frt/0

For further protection against spyware, the following programs are available free of charge to home users:
Ad-Aware
http://www.download.com/Ad-Aware-2007-Free/3000-8022_4-10045910.html?part=dl-ad-aware&subj=dl&tag=top5
Spybot Search & Destroy
http://www.safer-networking.org/en/download/index.html