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.
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I must admit that Process Explorer was not so helpful in this case. You can search for files and the tool will tell you which process uses them, but all I got was that "System" was using ntuser.dat. At least this gave me hint that it was probably a service which caused my problem.
Process Explorer basically does the same as the Windows Task-Manager, although it is much more powerful. You get running processes' hierarchical display that shows detailed information on how certain applications work. Usually I use Process Explorer when I am troubleshooting malfunctioning programs.
Of course, you can also kill processes or even complete process trees. But be careful! Process Explorer is not as cautious as Task-Manager. If you don't understand what you are doing, you will end with a blue screen.
There are two modes: handle mode and DLL mode. You can switch between the two using CTRL+H and CTRL+D. In handle mode you get information about the opened handles of the process selected and in DLL mode about the DLL files.
Another nice feature of Process Explorer is that it shows what TCP/IP connection a certain process opened. This is very useful if you're worried that there's a Trojan hoarse running on your machine that contacts its master. Right click on a process and then select "Properties". One of the tabs will show the TCP/IP connections. The others have other useful information about the process, like CPU/memory usage or threads etc.
If you have no idea about the function of a certain process, right click the process and select Google. In most cases, you will find out about the process' purpose this way.
Process Explorer also has a performance monitor which I prefer to that of Windows Task Manager's. A small version of the performance monitor is visible in the toolbar. Double click on it and a more detailed version will open.
If you like Process Explorer, you can replace Windows Task-Manager with it. Select "Replace Task Manager" under Options, Process Explorer will then open instead of the Windows Task-Manager. Don't worry; if you want to have your old Task-Manager back, you only have to disable this feature.