Why is my modem so slow and other things?

I’ve recently moved and after setting up one of my computers (there’s three in my home office now) I could only connect at 9.6kbps (at most) to Earthlink.

I checked the phone line, changed computers and modems (the speed went back to a normal 50+kbps), changed telephone lines, and changed dial up numbers.
My guess (well big hunch) after much tinkering was that the phone line was bad but one modem (the one built into my IBM ThinkPad) could overcome the bad connection but the external modem (Viking) used on one of my desktop computers could not.
I headed over to Staples – a really great place to shop and bought a new modem.

I could have bought a $100 brand name US Robotics modem but instead paid $60 for a Zoom modem.
I figured a modem, is a modem. Furthermore, the US Robotics modem did not even including a serial cable!! I installed the modem and now the speed is 48kbps – I can live with that speed. I have broadband in my Manhattan office so only use a modem to download email and do some web browsing. Although I am a technologist I only buy the technology I need.