As I had run out of ideas and the problem seems to be popping up more and more, I decided to put together a system and duplicate one of the machines that recently came down with the issue. I started going step by step with the software installations until I dug deeper into the machine to determine everything that had been installed from October 2013 to present. When I got to the Windows INF folder, I found that several of the drivers installed were from Citrix. These drivers are USB Video Redirectors. When I found this, I recalled seeing the Citrix Connector software running on some of the other problem machines as well. I uninstalled the Citrix software, made sure the drivers were actually removed and then restarted the computer. When I logged back into Windows, I plugged in my flash drive, which wouldn't work just prior, and it actually worked this time. I didn't have time to do any more testing Friday. So, I when I returned to work this morning, I reconnected the third monitor via the IOGear USB adapter and waited to see what happened. Sure enough, the display came up and worked just fine.
I then started looking for an older version of the Citrix ICA Client/Receiver and ran across a single article on the Citrix support site that made reference to this exact issue.
http://discussions.citrix.com/topic/338636-a-usb-device...
The suggestion was to contact Citrix Technical Support and quote LA4582. (I assume that LA4582 is an internal knowledge base article). I do not know what the solution is. However, I would imagine that it's to uninstall the current version (14.x) and install an earlier ICA Client/Receiver like version 12.x or 13.x. Or, to swap the ctxusbm.sys driver with an older version. I get the impression that the driver somehow adjusts/modifies the USB Hub Amperage readings from 500 mA to 100 mA and the XP OS then views that as an exceedance of the Hub's power.
I know that this has resolved my issue. I can't say if it will resolve anyone else's. Definitely check into what drivers/software have been installed. I never would have thought that the Citrix ICA Client/Receiver would have caused an issue with USB function. And definitely look past advice such as "plug in powered hub" or "you have too many USB devices connected" or "just upgrade to Win 7" or "use a USB Y cable" or "you have a hardware issue". Take some time and be methodical about your troubleshooting. You'll find the answer eventually.
By all means, if you have a question for me, feel free to ask. I'd be more than happy to help.