Many times, the problem is with Windows itself although it is fairly easy to this. Download the System Mechanic trial and use the internet tune-up wizard. It is free and the results are permanent even if you never buy the product. There are similar tools but I have used that one and it works. I have recommended it to people here and they reported back that internet speeds were up to 10x what they once were. There are some Windows registry keys that can severely limit certain types of broadband perfformance and tools like this simply adjust them to the right broadband settings instead of the dialup defaults that Windows tends to use.
There is also the issue of shared bandwidth. There are probably more users with standard service on your line than those with premium service and you are sharing bandwidth with all of them. Also, depending on your service, if you move a lot of traffic they may decide to limit your bandwidth (a dirty little secret they tend to not mention). I have dedicated bandwidth DSL for just this reason. When I told Comcast how much traffic I move a month, they actually hung up on me!