Identifying the Source of an Untitled Window

Sometimes, bothersome windows may appear without any indication as to what program created them. The best way to find such windows' source is to identify the process that owns it, and then figure out what product the process belongs to. My preferred procedure:

1. Download Sysinternals Process Explorer
2. Run the program and accept the EULA
3. Drag from the "Find Window's Process" icon (looks like a target) on the toolbar to the mystery window
4. Look at the application name and company name columns for the newly selected row, which is the process that owns the window
5. If those data aren't helpful enough, Google the process name

This also works for windows that don't even have title bars, like those that are just gray rectangular regions. If you want to poke harder at mystery windows, try Spy++, which comes with Visual Studio.

Inspired by this Super User question and my answer there.