The IE Plugin runs some OCX controls, so that might be due to a different version of .NET or some Windows libraries or to the security settings that, for some reason, do not allow to run ActiveX in that specific computer.
Apart from that, some other antivirus software resident in that computer, and not in the other computers might be blocking the execution as well.
If all the components are registered OK, you should be able to open qvp://servername instead of the Ajax AccessPoint, that will ask you for credentials.
Are all computers running the same version of IE 8?
Hope that helps.