Click the Windows "Start" button, select "All Programs," click the "Skype" folder and then click the "Skype" item. If the application is already open, you can also bring up the program window by double-clicking the Skype icon located at the bottom-right corner of the screen, within the Notification Area.
Click the "Tools" item in the top menu bar and select "Options."
Click the "General" item located on the left side of the resulting dialog box.
Click the adjacent "General Settings" item.
Look to the right side of the dialog box and uncheck the "Start Skype When I Start Windows" box.
Click "Save" to confirm the change.