To stop Speedster, right-click the Speedster icon and click "Stop Speedster"
The icon will now have an X over it to let you know Speedster is disabled

To start Speedster, right-click the Speedster icon and click "Start Speedster"
The X will now be removed to let you know Speedster is enabled

You can also start Speedster through the Start Menu:
Click Start -> go to Programs -> go to Speedster
Click Speedster Web Accelerator