Scanning the computer from an online location provides several advantages over the classic local scan. The most prominent one is that the risk of a virus infection of the virus scanner is not present and that the engine used by the scanner is usually up to date. A local virus scanner needs to be updated regularly to stay effective.
An Internet connection is needed on the other hand to perform the scan which sometimes can be a problem if the system is not booting into the operating system. A Live CD could help but many Online Virus Scanners demand the Internet Explorer which is obviously unavailable on Linux systems.
Online Virus Scanners can be used to get a "second opinion" without having to install another anti-virus software on the computer. It is probably a good idea to use as many of the virus scanners as possible if it is suspected that the system was infected by a virus. Below is a list of services that provide access to online virus scanners.
Bitdefender Online Scanner - requires Internet Explorer 4+
Eset Online Scanner - requires Internet Explorer
F-Secure Online Scanner - works only with Internet Explorer 6+
Kaspersky Free Virus Scan - browser independent, downloads roughly 25 Megabytes of files prior to scanning. User can select locations to scan. The scanner does not remove infected files.
McAfee FreeScan - requires Microsoft Internet Explorer 5.5+
Microsoft OneCare Live - requires Internet Explorer.
Panda ActiveScan - requires Internet Explorer or Firefox, does not run in Opera.
Shields Up! - browser independent but very slow and unresponsive currently.
Symantec Security Check - down or gone.
Trendsecure HouseCall - Java based scanner, works with Java compatible browsers.
Windows Security - Trojan scan that requires Internet Explorer 5+
File Scanners:
Avast Online Scanner - file size limit of 512 Kilobyte
Virus Scan - file size limit of 10 Megabyte.
Virus Total - email upload option, 10 Megabyte file size limit.