since I installed w3
i guess you didnt install it since you have this problem. I had the same problem time ago when i was lazy to install w3 (just copied)to my notebook so i was opening replays with some parsing program(or copying into /replays folder and then single player -> view replay) but then i tried sth right click-> open with -> browse programs (or sth) and then i found in w3 folder sth like war3.exe/war.exe (try all possibilities) and then it worked for me so will work for you too.