Another option could be "Comodo Firewall" http://www.comodo.com/ It's free unless you pay for their tech support, some nifty extra features, and remote troubleshooting.. It is mainly a firewall.. and an extremely intelligent and well-written one at that (IMO). It's default settings are decent, and you can fine-tune the engine for more/less security. It also comes with a "defense+" module.. basically it's a firewall but for your local computer. It mainly monitors for changes and such to prevent the installation of rootkits, trojans.. etc.. and you can scan your computer for malware + remove it as well. I find that Comodo has a relatively small memory footprint.. on default settings (vista 32bit).. seems like both the kernel-mode drivers and and Ui top out at about 60mb combined max (with defense+ enabled) and 30mb max (with defense+ disabled and only the firewall running). I've also found that Avast and Comodo work perfectly together.. so if you do not like Comodo's built in Defense+ mode you can disable it and just use Comodo's firewall + Avast's system protection. ClamAV may be your best bet however if you don't want an active scanner.