Windows 7 - Windows 7 Not Recognizing USB Devices Plugged in After Power-up

I recently upgraded my computer's OS from Vista 64bit (I think it was Home Premium) to Windows 7 Ultimate 64bit. Since the upgrade my computer has not be able to recognize any USB device that is plugged in after the computer has started--this includes an Western Digital 300GB Passport, a 2GB SanDisk thumb drive, a couple of SD cards, and my USB mouse. I have tried every USB port on the computer (2 on front, 4 on back) and all produce the same lack of result.
What's going on here? All of these devices were able to be dynamically recognized before I upgraded to Windows 7.

Thanks



EDIT: I unplugged my thumb drive to check it's brand while I was typing the original part of this post; Just now I plugged it back in and was preparing to restart the computer so it would be recognized but suddenly the computer recognized it. I unplugged and plugged it back in again and it is still recognized properly; I also tried another USB port and it is still being properly recognized. I plugged in a different USB device (the WD Passport) to the same USB port and it was NOT recognized. I also tried it on another USB port to be sure and it was still not recognized.
I'm not sure what this means but I thought it should be mentioned. I don't know if the thumb drive will still be able to be recognized if I unplug it, restart the computer, and then plug it back it back in after start-up. I would check this right now but I have some work I need to complete before I leave for my job in a couple of minutes.