No USB devices are recognized after plugging in SimpleTech external hard drive; pls help!

I posted this previously in the hardware section for Dimension 8300 and a friendly responder suggested that I repost it here. I unplugged a thumb drive on the front USB port on my Dimension 8300 running xp and plugged in a SimpleTech external hard drive that I had previously used on another compaq computer running xp. Although device manager tells me that all USB components are working, and although I can still pull power from the USB ports to power an MP3 player, the Dimension does not "see" the drive, nor any other device connected to any other USB port. I have done the following: (1) deleted USB hubs and controllers, then restarted, (2) deleted all drivers and reinstalled them, (3) disabled power management, (4) installed Dell's Driver Reset Tool, (5) reinstalled the chipset software, (6) reinstalled/repaired Windows XP, (7) tried various hotfixes that Microsoft has on its website for USB problems. Nothing has worked; the only hint I have is that when I reinstalled/repaired Windows I got a message that the computer could not find omci.sys, and while I could not find omci.sys on the Dell installation disks the file remains on my hard drive. Are there any other ideas out there as to how to fix this? Thanks very much in advance.