You can only have 1 ISP using a BT line at any time. What broadband company are you with? It is sometimes worth checking that your PC is not downloading any updates from the Internet and this could be causing a speed reduction.

I would also run a spyware-virus scans to make sure that nothing is on your machine. Make sure you do a thorough scan. These are quick easy things to check and will offer you benefit in speeding your system up.

If all is fine on your computer then it could be a few things such as:

1. How far from the exchange are you (if on BT line)
2. Do you run an extension lead in the house?
3. Are all websites slow or just certain ones?

There is obviously more that it could be, but check these first.