Nearly every security vendor offers a free virus scan on its Web site, but it's not always clear what they are, how they work, or why you might want to use them in addition to or instead of downloadable security software.
What are online virus scans?
The most common online virus scans are hosted on security companies' Web sites and use ActiveX technology to scan your computer, flagging any files that show up in the company's spyware or virus definitions.
Exact methodologies vary from vendor to vendor, as does coverage. Panda ActiveScan claims it detects rootkits. Kaspersky updates its definitions hourly; others once a day. Some, such as F-Secure, require Internet Explorers, while others, such as Trend Micro, also support Firefox.
When should you use online virus scans?
Online scans are best used for sniffing out low-level threats that don't impede Internet access. They usefully offer a second opinion while conveniently skipping the installation steps of downloaded programs, and are usually compatible with security software already installed on your PC. If you're concerned about higher-level threats, be sure to read these techniques for removing a Trojan horse.
The disadvantages of online virus scans
Most scans are just malware detectors that won't remove pernicious software unless you purchase the product. That's not necessarily a drawback if you're open to new removal software; the online scans test-drive the product's efficacy. (BitDefender is an exception, offering gratis removal services in addition to a free scan.)
You'll also still want to allot a good chunk of time to the scans--deep examinations can take hours.
Some of the better-known online scans include: