Some viruses are started when the computer is turning on because they are placed on the list of startup applications. If you plan to modify the startup application list you can use the "msconfig.exe" application. You can execute it the "programme run"( start -> run ) by typing "msconfig"

After you open this application select the startup tab and you can see list of available start up applications. Then you can disable them as your choice. After the mission it will required a restart to apply the changes.