Very Slow To Open Files From Windows Explorer - Ok Within Application

Sometimes if I open a file by double clicking it in windows explorer, it will sit doing nothing (and that explorer window is then tied up) for a few minutes! Then the file opens normally in the correct application. But opening the application first, and getting it from the file open menu, or dragging the file into the open application is ok. The problem goes away if I reboot windows, but can come back again later for no apparent reason.