It could be the effect of a memory leak of some program (Firefox is prone to this). If you are running Windows, type msconfig In the “Run” dialog. You will see a multi-tabbed menu, and among the tabs the “Services” one. You can order the list and eventually spot, among the non-Microsoft services, the offender. The “Tools” tab groups the tools that are used to monitor and log programs and events, and I would look also there. Windows has also a semi-hidden malware removal tool that you can launch typing mrt. It’s a wizard.