Best anti virus to pay for is ESETNOD32
Key Features
Smarter Scanner – Threats don't always enter your network in ways you expect. ESET NOD32 Antivirus inspects SSL-encrypted communication channels like HTTPS and POP3S and intelligently scans compressed files to find hidden threats other products miss. Proactive protection begins at the earliest point in system startup to ensure your computer is always secure.
Clean and Safe Email – Email scanning for Microsoft Outlook, Outlook Express, Mozilla Thunderbird, Windows Live Mail, Windows Mail, and other POP3/IMAP mail clients, ensuring your email is free of viruses and other threats.
Removable Media Security – Threats can enter your PC from removable media such as USB thumb drives. For self-running media, ESET NOD32 Antivirus scans autorun.inf and associated files when the medium is inserted, in addition to scanning any file on any removable device when it is accessed, or during a full-scan of the media. Power users can adjust ESET NOD32 Antivirus to perform additional levels of scanning on removable media.
System Tools – ESET SysInspector and ESET SysRescue simplify diagnosing and cleaning of infected systems by allowing deep scans of system processes to find hidden threats, and creating bootable rescue CD/DVD or USB drives to help you repair an infected computer.
Self Defense – ESET NOD32 Antivirus has built-in technology to prevent malicious software from corrupting or disabling it, so you can rest assured your system is always protected.
Usability Improvements
ESET NOD32 Antivirus has numerous speed, security and usability upgrades such as:
* Energy-sipping battery mode extends laptop battery life without compromising security
* Advanced Protection Status screen informs you of threat detections
* Information pop-ups are hidden when running full screen applications like games, video players or presentations
* Password protection prevents ESET NOD32 Antivirus from being uninstalled by strangers
* New interface and keyboard shortcuts simplify use for visually impaired users
http://www.eset.com/home/nod32-antivirus
Best free anti virus is Avast
Antivirus and anti-spyware engine
State of the art scanning engine provides reliable protection against viruses, spyware and other forms of malicious software.
Real time anti-rootkit protection
Unique "on access" technology checks all kernel-mode drivers that the operating system is loading for signs of rootkit behavior. This enables even unknown rootkits to be detected and stopped before they can do any damage.
avast! Community IQ
New Code emulator
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New Heuristics engine
Potentially unwanted programs detection
Boot-time scanner
Wake-up for scan
Scheduled scanning
New avast! Intelligent Scanner
New Behavior Shield
Behavioral Honeypots
File System Shield
Mail Shield
Scans all incoming/outgoing e-mails for malware. For MS Outlook, a special plug-in is used. Other email clients are protected by a generic POP3/IMAP4/SMTP proxy. New in version 5, the proxy also supports scanning of SSL communication (gmail.com, sympatico.ca, ….)
Web Shield
Scans all visited web pages and checks all files, pages and java scripts downloaded from the internet. Thanks to the Intelligent Stream Scan feature, the Web shield doesn't slow down your web browsing.
P2P Shield
IM Shield
Network Shield
Smart virus definition updates
New Fast application of updates
New Silent/Gaming Mod
CPU optimization
New Multi-threaded scanning optimization
Green computing
Reduced demands on the disk drive result in lower energy consumption.
http://www.avast.com/free-antivirus-download#tab2