Uh, yes, I would be very disturbed about this behavior as well.

I would not invite your friends over to sleep anymore. Go to their places instead so you can relax & have fun! Sleepovers are always a blast & that experience is waaaay too uncomfortable to let yourself have fun at home with your father around.

I do recall being paranoid in my youth....but as long as you're sure that you were awake & actually saw your father come into bed next to your friend....that isn't paranoia. I'd certainly advise you to stay away from his personal computer anyway, as it is his and your father deserves to have some privacy, just as you do. Do you have bedroom and bathroom locks? Perhaps using those could help reassure you while you shower/bathe/sleep.

Gentle hugs to you!