MALWARE BLOCKED, but can't find the source virus

One of our desktops keep on getting an avast! MALWARE BLOCKED message (see attachment). I’ve repeatedly run every avast! scan available, yet it doesn’t find any virus… I cannot see the path of “SkypeUpdateEx.exe”, the PC doesn’t even have Skype installed, and there are no unexpected applications in the start-up-registry settings, I am at a loss to how this virus is activated. There are no add-ons installed in any of the browsers, and no strange processes running. Please help?

What do you get if you click on “More details”…!?

Does this happen under all users, or just one?