Are people hacking into my computer?

hello its fair to say i am pretty dumb when it comes to technology…i use spybot, avast and the windows firewall to protect my computer..however I quite often download stuff from torrent sites and these are notoriously dodgy..also i live in a block of flats…is there any way to tell if my computer has been hacked? recently my connection has become really slow and i am wondering if someone is somehow breaking in and using it threw my pc…as far as i am aware my router and pc are cable only can i check this?.....am i safe? how do i tell..sometimes i buy cds and groceries online and i dont want people to know my card details ya know..or anything about me really