You can use the following one-liner in Form_Load to center your form on the screen on startup:
Move (Screen.Width - Width) * 0.5, (Screen.Height - Height) * 0.5
Unfortunately this is not perfect, as it does not take the height and position of the taskbar into account.
It should be possible to get the height of the taskbar with the
SystemParametersInfo, I'll try to post a more complete example later.