Over the past week I've seen increased chatter across various forums about internet connection speeds suddenly dropping over night for no apparant reason.

I too was affected by the same problem just over a week ago and at the time blamed the current weather. Having spent a couple of hour looking into a possible cause I discovered that a recent Windows Update had downloaded a new set of drivers for my Realtek network card. Upon rolling back to the previous version speeds suddenly shot back up to normal levels.

So if your router is still synch'ing at its usual speeds and your running Vista, check your network card and either rollback or download the manufacturers official drivers.