Online antivirus scanner gives you the option of an alternative virus scan without downloading a full product. This is not a replacement for full antivirus programs but an alternative second opinion. Each online antivirus has requirements. Most of them work only with Internet Explorer. But Trend Micro Housecall and Panda NanoScan work also with Firefox.

Periodic online virus scanning is a good idea for Windows users, even for people already using antivirus tools. Why? First, antivirus software is frequently slow to spot new threats. Take a gander at the daily “unrecognized” stats posted by Shadowserver.org, which tracks the performance (or lack thereof) of several popular tools in spotting new variants. That list currently examines the performance of several free programs, but the reality is not much different with the commercial tools. Just have a look at performance metrics and virus detection failure rates chronicled here and here.
Second reason follows from the first: If something nasty does make it past your security defenses, usually the first thing it will try to do is disable the active protection and update features in those tools. In such cases, you probably would not know about the infection unless you turned to a third-party program that is not already installed on your computer.
An online virus scanner should never be considered a substitute for an installed antivirus scanner. But if you suspect your installed antivirus is missing a threat or you simply want a second opinion, an online scanner can be a valuable adjunct to security. Each of the following online scanners are considered best-of-breed and can offer important adjunct protection.

BitDefender Online Virus Scan

Its features all required elements for thorough antivirus scanning and effective cleaning: scans your system’s memory, all files, folders and drives’ boot sectors, providing you with the option to automatically clean the infected files.

HomePage: http://www.bitdefender.com

Kaspersky Anti-Virus Web Scanner

Detects only, does not remove.

HomePage: http://www.kaspersky.com

Trend Micro HouseCall

Trend Micro HouseCall is an antivirus for checking whether your computer has been infected by viruses, spyware, or other malware. HouseCall performs additional security checks to identify and fix vulnerabilities to prevent reinfection. Very slow but very good one.

HomePage: http://prerelease.trendmicro-europe.com

Panda ActiveScan

Scans, disinfects and eliminates over 185,000 viruses, worms and Trojans from all system devices, hard disks, compressed file and all your email. Detects spyware. Antivirus detects the following types of malware: dialers, hacking tools, rootkits, jokes, security risks. Vista not supported.

HomePage: http://www.pandasoftware.com

Panda TotalScan

Detects only, does not remove. Capacity to detect any type of threat as quickly as possible thanks to its rapid scan. You will have a second opinion about the security of your PC.

HomePage: http://www.nanoscan.com

F-Secure Online Virus Scanner

Two options of desinfection: automatic or manual. This antivirus is very good one. Vista is not supported.

HomePage: http://support.f-secure.com

ESET Online Scanner (NOD32)

The ESET Online Scanner is a antivirus which you can use to remove malware. The ESET Online Scanner uses the same ThreatSense® technology and signatures as ESET NOD32 Antivirus, and is always up-to-date.

HomePage: http://www.eset.com

eTrust Antivirus Web Scanner

It can test also your removable disk.

HomePage: http://www3.ca.com

Windows Live OneCare

Unfortunately skans automaticly, no options to choose. For Vista users: beta version Service offers Safety scanner, Protection scanner and Tune up scanner. What does the complete scan do? The complete scan runs all three scans offered by the Windows Live OneCare safety scanner:

• The protection scanner checks your computer for and helps remove viruses, spyware, and other malicious software (also called malware). It also checks your computer for common open ports that might make your computer more vulnerable to online threats and hackers.
• The clean up scanner automatically checks your hard disk for any unnecessary files and gives you the option to delete them. It also checks your registry for stray data that may have become obsolete and gives you the option to delete it.
• The tune up scanner checks your hard disk for its level of fragmentation and, if it’s more than 10% fragmented, asks if you’d like to defragment it.

HomePage: http://safety.live.com

a-squared Web Malware Scanner

You can test your system with the a-squared scanner for Trojans, Backdoors, Worms, Dialers, Spyware/Adware, Keyloggers, Rootkits, Hacking Tools, Riskware and TrackingCookies. Whitlist of your files is available. It runs in 4 options: Quick / Smart / Deep / Custom.

HomePage: http://www.emsisoft.com

Ewido Online Scan

Fixes spyware and malware.

HomePage: http://www.ewido.net

Symantec Security Check Online Scanner

Detects only, does not remove. Don’t even try.

HomePage: http://security.symantec.com

AhnLab MyV3 Scanner

Automatically updates the latest engine upon connection. Antivirus scans boot, memory areas, and compressed files. Can be configured to either basic or advanced mode.

HomePage: http://global.ahnlab.com

Authentium Command On Demand Scanner

Simple scanner. Can be configured.

HomePage: http://www.commandondemand.com

CounterSpy Scan

Detects only, does not remove. Antivirus detects perfectly, nice reports.

HomePage: http://www.sunbelt-software.com