Try limiting the upload rate of your torrent client to no more than 80% of the maximum upload speed of your internet connection
Usually this happens when BT uploads as fast as it can, drowning and killing off anything else your computer is trying to say to the internet like "log me in, eoff forums"

If 80% is still too high, keep reducing it by small amounts and try again
Also, I generally find wireless not too happy with large amounts of traffic which torrents like to generate
Also, since you're using Windows Vista, who knows what crap DRM they've implanted to cripple the machine if it's doing anything slightly suspicious
