Monitor file system, Registry and process activity
If you think Process Monitor is just a more complete alternative to the standard Windows task manager, you're wrong. This is a exhaustive control tool, mainly intended for system administrators, which tracks all the activity in Windows file system and Registry in real time.
So when you run it (no installation required, but it does ask for administrator rights) you'll get a constantly increasing list with all the processes taken place in your computer in real time.
In fact, there's so much information to absorb that it makes you feel a bit lost.
Process Monitor includes many tools to manage this list: filters, a nice search tool, extended information about each process and many more.