This is usually down to badly written software, spyware, viruses etc - like they try and open a port or connect to something then when your firewall refuses them, they keep trying, and keep using up all your processor.
A decent virus and spyware protection system should get rid. I'm prepared to be corrected, but I quite like Kaspersky, there's a free month trial - if it fixes the problem then maybe it's worth registering. I'm using it at home, and it's got rid of so many little niggles, it kicks AVG right in the nuts. AVG often causes the problem that is affecting you!.