how do i make my Win7Pro computer secure as administrator i.e. private and secure from other computers/users?

My desktop (Win7Pro) is connected by cable to my router/modem (netgear DGN2200) I share my connection with two other users wirelessly in my complex/units.

the other day one of these friends brought his laptop up to my room and hacked straight into my computer

HOW CAN I PREVENT THIS——-PLEASE

Posted in Uncategorized