Process Explorer
Are you still using the default Task Manager? That's so 2006. All the cool kids are using Process Explorer. Microsoft bought the product a year or two ago and it's free for the taking.
The first thing you'll want to do when you fire it up is click Options > Replace Task Manager. Now when you open the task manager in Windows, Process Explorer will automatically open in it's place.
I find it super useful for those pesky "Cannot delete file because another process is using it" messages. Fire up Process Explorer and choose Find > Find Handle or Dll. Type in part of the filename and search. You'll see which process(es) are using the file.
There are lots of other great features, but exploration is part of the fun so I'll leave that up to you.
