If you are unable to start a Skype conference call, this may be due to add-on software or hardware drivers interfering with Skype.
One known example is the software that gets installed with the Logitech QuickCam. (See this link).
If you suspect add-on software or hardware of causing an issue such as this, try uninstalling your add-ons one by one to find out which one is causing the problem.
Thanks to Kyle Jones for making us aware of this.