I had two computers made up at a local computer shop. They are identical except for the modems. The shop ran out of their usual Trendware modem, and substituted a U.S. Robotics modem bought retail for one of them. Both machines now sit on the same desk, dialing the same Earthlink number on the same phone line, to which they are connected in series. The U.S. Robotics modem reports "57Kbps" and the Trendware modem reports about 24Kbps at my home, but about 50Kbps in the shop (so they wouldn't replace it under warranty). I ordered the same model U.S. Robotics modem from Overstock.com and it also ran slow, even though the driver it installed was the same. That modem broke almost immediately anyway. While playing with modems, I tried installing the good, fast U.S. Robotics modem from my machine in the other machine, and it reported the same slow speed. I also switched around the phone cords, using a fresh cord and setting it up so the slow modem wasn't downstream from the other modem. Nothing helped. The driver for the fast modem was from 2001 and hadn't been updated.

What could possibly make these two otherwise identical computers dial up at different speeds, but without the difference being apparent in the shop? The only differences between them are the CD-ROM/R/RW drives and the software installed in each. What can I try next?