Optiplex GX520 USB keyboard issue

Hi

just got a refurbed Optiplex GX520 small form factor desktop with XP/SP3 & cannot get any keyboard to work with it

there's no ps2 port in this model only usb, im using a ps2 to usb converter & no matter which usb port i use it tells me 'device not recognized'

in one of the rear usb ports i sometimes get a message telling me a new usb device is installed but may not work properly, however this is always immediately followed by the same 'device not recognized' message

ive tested all the usb ports with my usb mouse which works in all of them ok so i know theyre not faulty

i have all the latest product drivers for all 3 keyboards i've tested.

Ive tried updating the drivers directly through device manager / running the new hardware wizard / restarting & plugging in the keyboard in at various times.. but nothing

theres no specific keyboard drivers on dell's download site, just bluetooth & mouse drivers

cant find anything about this online either.. please help!

thanks in advance

Guy