in sysinternals you can find many tools,
this tool: Process Explorer is one that I found useful for my last week.

I am using ESRI product called: ArcGIS Server, which is an of the shelf product with huge API and large architecture, I made some calls from the client to the server, their Map control show an Image from file system, somehow the file is locked: cannot be deleted, cannot be changed., my bigger problem that other users which using the same control and trying to watch the map cannot enter it. I wanted to find which one of the processes locked my image.
Process explorer come to the rescue: you can find more details about its feature here.

Do not afraid of debugging the unknown.