If your USB ports seem to be malfunctioning or not working on your Dell system after a fresh install of Windows or significant system change, you may need some drivers.

[edit] Steps

Download the proper drivers from the Dell web site. The best way to do this is to find the Service Tag number on your Dell and enter it into the appropriate box on the "Service and Support" >> Driver Downloads page.
You will need to install the chipset driver and if available, the USB drivers too.
If you need other drivers as well, the hardware can be difficult to identify, so check the back of your system for some basic information. If you are plugging in your monitor, network cable, or speakers into a port that is located within the rectangular IO shield, then you will want to find the appropriate driver with "integrated" in the name. This will probably help you narrow down which driver to use.
There is a utilitiy called "Unknown Device Identifier" which should be easily found by googling. It is put out by a group called "huntersoft". This can help you identify devices that may or may not have come pre-installed in your machine. It has helped me many times find obscure drivers.

[edit] Tips

Next time you need a new computer, have one custom built by a local business instead of getting a "name brand".
The name brand computers rely heavily on proprietary software and hardware that makes upgrading or repair VERY difficult.
Sometimes removing the side panel of your case is necessary to identify various types of hardware that may be installed. Exercise caution not to accidentally remove or loosen any cables or cards.