You can also manually terminate tasks. Things such as mwsoemon will interrupt defrag.

The method for listing currently active processes (tasks) varies between versions of Windows and NT. On Win98, ctrl-alt-del brings up a box which lists them, but I suspect that on other versions, you will get only the option to close down the machine.

A small icon in the task bar should give you a list of processes in NT 4.0 / 5.0 / Win2000.