1. Open the program and go through its menus, looking for the setting which
starts the program with Windows. Disable this setting.
2. Remove the program's shortcut from the Start menu's Startup folder.
3. Uninstall the program.
If you are unfamiliar with the Windows registry should not attempt to edit
it. You can unwittingly damage your system.
If the program in question is a virus or spyware, you will need an antivirus
program or a spyware removal program, respectively, to successfully prevent
the program from starting with windows.
Rocky