Turns out you were right on the nose, Halo - turns out WinXP was choosing the connection. This is how I found out.
I went to BestBuy to purchase a range expander, but the tech talked me out of it and said I should get an access point. Soooo, I brought it home to install it and found out part of the problem. It wasn't recognizing the router. The connection I was getting was through the computer's net adapter, NOT the DI-624. So after spending an hour on the phone going through the configuration process with the VERY patient Verizon tech, we got the main computer to connect through the D-Link. We had to upgrade the Firmware and reset the device. I was left to network the other computers myself. Using the Access Point (which gives an "Very Good" signal - 4/5 bars), I can now get this speed on the kids' computers::
:::.. Download Stats ..:::
Connection is:: 6790 Kbps about 6.8 Mbps (tested with 12160 kB)
Download Speed is:: 829 kB/s
Tested From:: http://www.testmy.net/
Test Time:: Wed Apr 13 2005 18:39:24 GMT-0700 (Pacific Standard Time)
Bottom Line:: 121X faster than 56K 1MB download in 1.24 sec
Diagnosis: Awesome! 20% + : 158.27 % faster than the average for host (verizon.net)
Validation Link:: https://testmy.net/stats/id-3452SYO6Z
Which is not bad. I get almost a full 15 Mbps on the main computer. Turns out the kids' computers were getting their signal through some wireless connection in the neighborhood - not through our router. Which is the reason I went through the steps to secure our connection. Only problem I am having now is that on the kids' computers, the connection gets lost and we constantly have to click on the "available wireless connection" and connect again. B ut, at least we're getting somewhere now.