This is a common bug (and still not fixed) with Warcraft 3 and Windows 7 where if you try to open replay file it will only start WC3 and not start replay.
This will also work if you have re-installed system and WC3 replay files are not associated with WC3 (Windows XP+).
Open Notepad and paste:
"D:\Games\Warcraft III\war3.exe" -loadfile %1
Replace path to war3.exe if you have WC3 installed elsewhere
Now save file with .bat extension (example: wc3replay.bat).
Now go to replay file, right click > Open With