This happens with Windows 7 users who have set their displays to use the Windows Aero theme. This will not prevent you from using Screen Share. The notification in the task bar will state "Windows Aero theme is being disabled and switched to Windows basic theme".
When Screen Share is started, the screen will go blank for a second as that switch happens. When Screen Share is stopped, the reverse process happens, and your display automatically switches back to the Windows Aero theme you were using initially.