Online scanners download a virus/malware definition database and an ActiveX file that allow them to run in the Internet Explorer browser; the download and scan can take a long time, even with a fast Internet connection.
Scanners are practical when you are looking for verification that a problem has been fixed or that a computer is infected. Unfortuantely, very few scanners will remove malware. Most importantly, since two antivirus programs should not be installed on a computer at the same time, online scanners are safe to use with your current antivirus program.