You don't is the basic answer. In a case like this you either start looking for a new system or trolling ebay for someone with the same system that they're parting out for reasons other than something is wrong with the motherboard.
You can try contacting Dell, but unless the system is under warranty, they aren't likely going to do anything.
All of a sudden none of my USB ports are working on my Dell XPS 400 (XP, not Vista), circa 2006. Power is fine and the hard drive is spinning.
How can I even get into setup mode or boot from the CD? I don't have any alternative for keyboard/mouse use since I only have USB ports on this computer.