If your in IT and haven’t read Mark Russinovich’s technical blog then you’re missing out. This guy knows his stuff and in this particular article “The Case of the Slooooow System” he takes you through high level troubleshooting techniques using Process Explorer.
 

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Did you know – you can find out who is logged on to a particular PC on your network with the whoison.exe utility.
To use the utility, save it to your local hard drive or your network drive. I have it saved to the root of my C: drive for ease of access.  Run it from a DOS command prompt, (see example below): Type C:\whoison \\computername
 
Use the /h switch to get a history of users on the PC:
whoison /h \\nameofcomputer
Use the pipe command to send the results to a text file:
whoison …