Unable to open the Virus Scan Provider in Visual Admin

Hello, dear experts!

[SAP WebAS 6.40 SP13 Windows]

We have downloaded and installed the latest Anti-Vir dll as a virus scan provider (they are officially certified provider).

After I tried to activate it gave me error message that the version of anti-virus software is incorrect.

I restarted the SAP WebAS and since then I can't even get to open the Virus Scan Provider service in Visual Admin anymore.

The Visual Admin complains on the status line:

java.lang.ClassNotFoundException: com.sap.security.core.server.vsi.gui.VSIRuntimeControl.

So I can't even change it back now.

Please help!