you are between a rock and a hard place...
I will assume that your computer is password protected...
right click someplace on your desktop, go to the "Screen Saver" tab, select
your favorite screen saver and place the check in the "On resume, password
protect", then click OK
there is a problem with this, you will have to set a time short enough to
give you protection, but if you sit a think a little bit, you will have to
keep typing your password.