Security threat caused by Dropbox?

Recently I received a security threat alert triggered by Dropbox, as seen in the screenshot below:

What happened here? Or this is a false alert from Microsoft Security Essentials?

This is nothing to do with DropBox - even your screenshot shows what the problem is. I'm guessing one of your computers with DropBox does not have an up to date antivirus, so it didn't pick up the infection, but when DropBox replicated those contents through to this computer it spotted it because of MSE.

The upside - your local machine is probably not infected, as MSE picked it up and suspended the file.

You will need to clean your other machines that share the DropBox though.

Submit a ticket or contact [email protected].

It seems one of your synced computers was infected with Ramnit.B and that (the file) synced with your Dropbox. Ramnit.B is not Dropbox specific.