The latest one available on your download page at dell support but I doubt that will correct the problem. Run the Dell Diagnostics to see if you have failing hardware. The Dim 5000 is pretty ancient now and is not worth spending anything on. Consider buying a new computer.

Which USB ports are failing? All of them or just some?

You might inspect the insides of the ports for bent pins that are shorting out: those can physically damage the motherboard components.

I have seven USBs and all have stopped recognising any device that has storage, e.g. pen drive external drive and MP3 player but the ports still recharge devices and my webcam and keyboard work as normal. I have run Dell diagnostics on the USBs and it only said that some had failed. All the ports failed at around the same time so I don't think it's a physical damage to port issue. I tried enable/disable on all USBs and driver update said no better driver could be found. I need XP for my music studio and I had a new hard drive fitted a few months back so I'm not considering junking it yet. I didn't think it was a BIOS issue but the Dell support page suggests updating it for USB issues. Perhaps it's a chipset problem?

Since you have nothing to loose, back up your data, because unsuccessful BIOS update might brick/damage your motherboard, go to support page as Mary G advices, get the latest one and update following instructions provided. If no help after this, you may try reset your CMOS by removing motherboard battery (remember which side is up), leave it out for 15 min and place it back. If this not going to help, reinstall windows and after this there is noohing left to do.