First, just be glad you're not going through the USB 3 hassle I am! Next you can try going into Device Manager and deleting the USB entries followed by a reboot. Windows will recognize the USB ports and then load the latest drivers. That works for some. Next, sometimes the contacts in the USB slots wear down to the point where they don't work so well. For around $20 you can buy an expansion card that adds more USB ports and try that. Just be sure to check whether you need PCI or PCIe ones. Newer PCs use PCIe.
I assume you're not having other problems with this one troubling PC, but if you are, you might want to boot to Safe Mode and make sure you don't have a virus or other form of malware.
Ok here is my issue, we have multiple computers in the house, 1 using Vista still (haven't spend the money to upgrade the OS and I am happy with it as it is being a laptop) and the others use WIN 7. Now recently one of the WIN 7 computers has stopped recognizing USB Hard Drives all together, they get power but that's it, no attempt to install or ever try to recognize the device is even there, but all other USB devices work normally. However on all of the other computers these same USB Hard Drives have no issues at all. I am about at my wits end trying to figure this out so now I come here for help.