I had the same problem myself on two computers (home and office). The answer for me the first time was to visit the modem manufacturers web site and download the latest modem drivers. The second time, brand new computer, the internal (infernal) modem decided to work slowly. this was easily detected by transplanting the office modem to home. I also now run a program eBott which I downloaded from the web somewhere, cost me $10 I seem to remember.
Try clicking on the computer icon at the bottom right of your screen when you are surfing to see what speed the modem says it's running at - you may have an iffy phone line- if it sounds hissy to you, it will be unreadable to the modem. Go into My Computer, then into Control Panel and then into Modems and check that the modem description matches yours. Delete any modems that don't match up, your computer may be using the wrong drivers. click on Diagnostics, then click on the com port icon associated with your modem, then on More Info, wait a while for the report, check that there are no "Failed" reports. look at the modem settings, check that you have the correct driver loaded for the modem - haven't by any chance swapped the modem for another without telling the computer have you ? You may also find that the driver is set for a standard modem, not the particular one you are using -may have been set up without the specific drivers which may be on a separate disk. Try re-installing the modem if all else fails !
Good luck ! Frustrating isn't it ?