Part of the problem may be that if you are not running a firewall you may have uninvited guests downloading programs from your hard drive.
I would suggest that you download the free version of zone alarm and set the security to high level, pop up alert when someone pings your modem, and "Internet lock" after a X number of minutes if you have not been at your PC.
tc can tell you anything you want to know about the program. He showed it to me.