Dr.Web Anti Virus for Windows

From Doctor Web:

Exceptionally compact and fast anti-virus solution for PC. Multilevel protection of the system memory, file system all removable media against all types of viruses, rootkits, worms, trojans, spyware/adware, dialers and other malicious programs. Virus definitions automatic updates at hourly bases going up to twice per hour frequency at periods of high malware submissions. Basic AV functions (deep system/hard disk on-demand scanner, on-access 0-ring monitoring, highly-tuned mail-filtering) combined with proactive defense capabilities and new generation heuristic analyzer of Dr.Web anti-virus allow to install and forget about malware-related threats.