Bear in mind that even the most powerful PC is limited by the internet connection.
If your connection is slow or there is a lot of traffic online at the same time as you, downloads will be compromised. Think driving a Ferrari on the M25 at rush hour !
You can check the speed of your broadband link - there are a number of websites offfering this but here is the one on the BBC site -
http://news.bbc.co.uk/1/hi/technology/7416471.stmIt is unlikely your results will be close to the service you are paying for - e.g. if you pay for an 8Mb service then don't expect the results to be up there at 8Mb! However if the result is a long way short of that then you have a good case to shout at your ISP about poor service. Test the connection over a number of days though to establish some consistency before you challenge them.
You havn't mentioned if other programs on the PC are slow - is it only the internet? If everything is slow, i.e. internet and all other programs, the problem would appear to be the PC itself but if it is only when you are on the web then the most likely culprit is your broadband connection.