To match a soundtrack to a mixed audio/visual track you need to be able to see both soundtracks down to 1 frame, really. And see a reasonably good size soundwave. A 'clap' sound might well do at at less than one frame, but... The clap can be from a clapperboard or just hands, or any short sharp sound. WMM just doesn't have enough magnification. I suppose you could open both the video file and the sound file in a sound software like Soundforge (or maybe the free one Goldwave?) and sync them there.
A clapperboard is really useful where you have scenes shot on more than one camera, so you know instantly which scene you are comparing. It's the info on a clapperboard, as well as the noise, that you need it for.
Win XP, Athlon 1.8, G force 3 T, 1 60gig drive + 1 250 gig, Pinnacle Studio 9 plus mostly, Premiere 6, and Vegas 5 sometimes, Soundforge, Cool Edit.