Is it a device that requires installation of driver software before you plug
it in?

Failing that, an approach I use for non-recognition of USB devices that
had previously worked is to delete the registry keys under

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\USB

which start with Vid_

If you don't feel confident with that, backup the registry, or that part
of the registry OR simply rename those keys as xVid_ or similar.

This may affect other USB devices that you use, but in my experience this
simply means that they will take a little longer to recognise the next
time they're plugged in.

In addition to Mike's response, was this MP3 player plugged in to
your PC when you reinstalled XP?  Or, did you do a Repair install of XP?  
When you reinstall an OS (Operating System) disconnect everything
except mouse, keyboard and monitor.  If it was not connected,
then you already have a clean Registry and the player should not be listed.

Re install the software that came with it. When prompted and not before)
plug in the USB cradle.

Cheers,
Jerry