Intrusion Removal?

Hey guys! This is my first post so bear with me please.
I noticed recently noticed that my computer was at risk because of an "attack" on my computer that somehow wasn't detected by my Norton Internet Security product. I also noticed that my "Intrusion Prevention" was turned off! I tried turning it on again but it turned itself back off. I've run a full system and nothing was detected.
The problem is fixed now and my computer reads: Secure.
The only thing I would like to know is how do I get rid of intruders on my laptop!?

James