Need a little more info Elizabath..... I know they say that size doesn't matter hehe.... but what is the size of your HD, and how much space is left?
Following that, before suspecting the ISP, how is it running when not on the internet, ie. just using word, or pages etc?
If it is running slowly then, assuming that your caches, etc aren't loaded to the max, that you haven't flooded it with startup items, your desktop isn't full, and your trash is empty, you could always do a clean install of your leopard or snow leopard, and then a clean install of all the software that you have on there, and then migrate your documents, pics, music, etc.
Taking it a step further, which is tedious, and depending on how comfortable you are with it, you could do what I did....
Clean install of OS X and installed 3rd party software, and moving over the keychains, app support and preferences, and everything else necessary to have all the applications running where you left off. For instance, firefox with all the bookmarks and add ons, mail up and running etc.
When I took the last option, I gained a couple of gig more space, which means that I also cleared a lot of residual prefs etc that I had left over from old applications, as well as conflicting prefs for apps that I still use.
This sped things up significantly for me, but took a few attempts to get it right, as a lot of files needed to run applications as they were, were located in places that I didn't suspect they would be.
So, I guess, a little more info is needed unless I have answered your question already.