另一台电脑把带宽占去了

I believe Bell Sympatico offers 3M bit/sec bandwidth, or 370 bytes/sec. Your PC that downloads movies takes 370~ 400k bytes/sec, which is about the same as your total bandwidth.

The solution of your problem is put limitation on your movie download bandwidth. Say, use 300k at most so the other PC can still brower web with fast response. You may find such option in your downloading software.