How can I determine whether a specific file is open in Windows?

Use Use Process Monitor(Procmon) from the Sysinternals Suite, Click on Find Handle or Dll (or hit Ctrl-F) , type in the name of the file you are searching for, and you end up with a list of processes with that file open. 
http://stackoverflow.com/questions/15708/how-can-i-determine-whether-a-specific-file-is-open-in-windows