Regardless of the OS, some programs insist on loading themselves every time you start your computer. The most friendly ones will have a configuration option, while others will use the Startup folder. Unfortunately some are even more sneaky.
All but the very worst offenders can be managed through msconfig, the same program we used with Windows XP. Go to the Run... dialog and type in msconfig to launch it. If Run... isn't in your Start menu list, try searching for it. This opens a System Configuration dialog box.
Within the dialog box, go to the Startup tab. Search through the tab and uncheck any programs you do not want starting up each time you reboot. You're prompted to restart your computer, but its not required. Just manually kill the ones that started the last time, and next time they wont come back.