If you're looking for freeware, good and eating minimum system resources AV solutions then try:
Bitdefender Antivirus Free Edition
Spybot - Search & Destroy
Malwarebytes Anti-Malware
Microsoft Security Essentials
VirusTotal - Free Online Virus, Malware and URL Scanner - free online service that analyzes files and URLs enabling the identification of viruses, worms, trojans and other kinds of malicious content detected by antivirus engines and website scanners.
VirusTotal Uploader - handy little tool that allows you to send max.32MB files straight from your disk to VirusTotal online scan using right click's SendTo system menu.
