Sadly, broadband speeds are dependent on many factors, and always remember that a "2mb service" actually means "up to 2mb service". To get the full 2mb consistently, you would need to be within half a mile of the exchange, on new copper wires etc etc. ADSL is rate-adaptive, so renegotiates the speed on a regular basis to achieve the lowest error rate.
There is also the issue of "contention" - i.e. how many other people in your area share the data feed at the exchange. Depending on your service provider, this could be as high as 50:1 - i.e. for every data pipe in the exchange, there are 50 users on it. Obviously, when everyone is on in the evenings, downloading their videos etc, your service will crawl due to the shared bandwidth there.
Assuming that you are using an external, independent modem (i.e. BT Home hub, stand alone router/modem) and not one of the fantastically awful USB "modems", then the issues is most likely with your service and not the PC - however, if you have lots of services (i.e. anti-virus, anti-spyware, personal firewall, yada yada yada), there is a good chance these might slow down your system depending on how they interact (are they scanning everything as you click on a page), or if they are updating themselves etc. (And no, I am not suggesting you remove them all, it's just an area to be aware of
)
If you are using Internet Explorer 7 or 8, then these browsers are also known to be fantastically slow (to do with the amount of personal data they store about you and your browsing habits ... ), so it might be worth trying Firefox or Chrome to see if your experience is any better.
Also, make sure you are up to date with all your microsoft updates to ensure you don't have anything nefarious on your PC that is working away in the background sending out viral e-mails etc ...
Anyway, hope that helps!
but just thought I'd mention it like you do 
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