Most of the time when you can't copy text from a Windows application using right-click, Ctrl+C will get the job done. Unfortunately, some fields can be stubborn blighters, not allowing any sort of copy/paste action, in which case you need a utility like Textify.
RaMMicHaeL's Textify is about as minimal as programs get. It allows you to set up a hotkey combination that when used, will transform a uncopyable Windows text field into one that has text highlighting and copying.
Why might such a utility be useful? Error messages for one, as Windows' native dialogue box doesn't let you copy the text it contains, which makes diagnosing the problem via a Google search harder.
Sure, it's not something you'll be cracking out all the time, but it's a nifty tool to have in your troubleshooting toolbox.