I keep hearing this is a path issue with cygwin. It is prevent emacs from working within my cygwin. When I execute find on the cli (not bash/cygwin) I get the same error not matter what I type. I've read this is a problem with path creation within cygwin and that it should be prepending itself to the path. As you can see it is doing that.
Here is my /etc/profile
PATH=/usr/local/bin:/usr/bin:/bin:$PATH
export PATH
Problem is that as everyone else stated, emacs is using find.exe provided by windows. To change this, you need to change your %userprofile%.emacs file.
As nobody else states (even faq!), this file is not created automatically anymore. Go into Options > Save options the mini-buffer (one line at the bottom of emacs) will tell you where the file is being written to.
Go in there and add this line (You've installed cygwin at c:\cygwin, right?):
(setq find-program "C:\cygwin\bin\find.exe")