There are several types of intrusion detection tools out there, a popular one of which is firewall.
A firewall can be either software-based or hardware-based, and PC users usually go with a software firewall and large companies go with a hardware firewall.
Several months ago my PC was acting strangely with ads popping up everywhere. A virus cleaner didn't cut it. Somehow I found this firewall software on download.com and decided to try it out. It is so easy to use and since I've run Sygate my PC stopped giving me problems!
I am sure there are other great firewall programs but Sygate is definitely one of them. To download it, go to download.com and perform a search with keyword 'Sygate'.
By the way there are several great free antivirus software tools. Here's one of them called AVG Antivirus Free Edition