There are several ways, but if you are not confident I strongly recommend a useful little programme that makes it easy to stop programmes, but just as easy to start them again if you need to.
You could also check the folder called 'startup' in your start menu - if there are shortcuts to programs in there, they will automatically start when you switch your computer on.
Click start, then run and type msconfig in the box that appears and press enter.
In the window that opens click the startup tab. Scroll down the list until you find a reference to the program you want to stop running booting up. Untick the box and then click Apply at the bottom.
When you exit this window it will ask you to reboot your computer - you do not have to do this now. The next time you reboot the program will not automatically load.