I'm afraid that I'm not able to tell you exactly how these online tests work.
But I can tell you that it does not use ActiveX as Cowabanga states.
You're wrong, he's actually right. The online scanner from Trend Micro certainly does use either the ActiveX based core engine or their Java VM Core engine for scanning your PC online.
I'm sure most online virus scanners do, as this is the quickest and most reliable way.
