If you're still using the classic Windows File Manager (you can still access it by running winfile.exe from Windows 95 and Windows NT 4.x) following two short cut keys will help you to quickly select and deselect all files and directories:
Select all files and directories:
Click on a file or a directory
Press Ctrl+/ or Shift+/
Deselect all files and directories:
Click on a file or a directory
Press Ctrl+\ or Shift+\