Process Explorer from Microsoft is a must have utility created by Sysinternals that can put the Windows Task Manager to shame.
Like the standard Windows Task Manager, Process Explorer provides visual information about the various software programs, Windows Services and other processes running on your computer.
However, the main advantages of using PExplorer over Task Manager is the intuitive tree interface and the powerful search features to help you track down any culprit process or program that's eating up the CPU power or secretly trying to communicate with an external website.
The program names are displayed along with the standard icons and descriptions making it easier to identify them among hundred of other processes.
Process Explorer will also show a list of DLL and OCX files associated with any process running in the background. You can either terminate a single process or the entire process tree.
You can also capture a snapshot of the current processes in a text file. There's also a wheel which you can drag over any open Windows on your desktop and Process Explorer will show you detailed information about that process.
Just keep Process Explorer running in the background, minized to the system tray. When your system gets slow, fire this program and track the culprit process - either kill it or lower the priority levels.
Process Explorer | Download procexp.exe (no install required)
Microsoft Process Monitor - Regmon and Filemon in one tool.