Tools we find useful on a daily basis:
Free Online Virus Scanner - http://housecall.antivirus.com - Why we use it: almost all new smart virus's infect the virus scanner first in order to circumvent the virus's detection. We use Housecall when we suspect a virus but the installed virus scanner does not report any infections.
Free Downloadable Virus Scanner - http://www.grisoft.com Why we recommend it? Because a free virus scanner with regular updates are better than none at all. Almost 50% of all systems we service have either no virus scanner installed or updates over a year old. Out of the remaining, 45% have not had an update in the past 30 days.
Free Firewall for Personal Use - http://www.zonelabs.com Why we recommend a firewall? This will protect you from most of the Internet born worms floating around. It will also protect your computer from other hostile communications.