Buzzard, I assume you're using a PC (as those Mac fellas always like to say "I have a Mac"
) but not sure what OS.
Maybe try booting into safe-mode-with-networking, and retry the speed tests. Do you get better results in Safe Mode? If so then perhaps it's software that loads in the background in a full Windows boot.
I also recommend downloading
ProcessExplorer from Microsoft Technet. Some programs are designed to hide themselves from Task Manager (at least in WinXP, anyway), but ProcessExplorer does a much better job at showing what's running 'under the hood'.
Once you have a complete list of what's running, then you can selectively kill a process, do a speed test, kill another process, do another speed test, etc, until you find the culprit.
If the problem persists after you've stripped Windows down to just the essentials then it's looking more like a hardware issue (or maybe a rootkit virus?). but as you say two computers are having the same issue so hardware doesn't sound likely.
Oh, and have you run the same speed test in two different browsers?