Devices like Mobile Phones & Portable Audio/video Players (with USB connectivity) are called MTP devices.
Did you connect 'Mobile Phone' or any 'Portable Audio/Video Player" in your USB Port?
If yes, you will get this balloon in Windows Task Bar.
Upgrade Windows Media Player to ver 10 or higher to stop this 'task bar balloon' and install MTP (Media Transfer Protocol). Click here to know what is MTP