My girlfriend and I recently bought 2 used Dell Vostro 1520s from a company that's going out of business. They were sold completely wiped, so we installed Windows 7 on both of them, and they work fine except for one thing: none of the USB ports work. From what we can tell, they're not even getting power, as nothing that we've plugged in has even lit up.
We've tried installing every driver and update we could find but with no success. We've looked in the device manager, and everything claims to be working fine. The computers seem like they're in good condition, and it would be strange that the two we picked (they had about a dozen for sale) would randomly be broken in the same way.
Well, after 5 hours of frustration, your suggestion solved it in 5 minutes. That was totally it, no idea why it would be disabled by default, though...
For anyone in the future who may be wondering about this, basically just restart and hit F12 (or whatever key it tells you) to enter the BIOS. From there, there's an "Advanced" tab that lets you Enable or disable the USB ports.