I suppose that many sysops already know Sysinternals Process Explorer. Although I often meet system administrators who never used it. As it is one of my favorite tools, I'd like to introduce it now. Yesterday, I used the Process Explorer to find out which program used ntuser.dat.
If I ever get my hands on a windows computer, I am sure to install a whole suite of applications from sysinternals. They are amazing utilities, and being a sysadmin I need the most information I can get about the system I'm running. These tools can help you see what programs are running, what programs have tcp ports open, and so on. Process Explorer is just one of these amazing tools.