Well, if you can get into the BIOS, which I'm assuming is F2, then take a look at the USB Controller Settings. It should be set to "ON" so USB devices will be supported in an operating system. It should be "ON" by default really.
If it is in fact on and you still can't use the USB devices at OS. Let me know.
Also, there are 4 diagnostic lights on the front of the case. If the PC boots with no errors at all, there should be no lights on. If there are errors like "USB" errors, there should be some lights still on. ( Lights 2 green, 3 green and 4 green )
Here's the service manual for that dimension. Use it as a reference.