Far as I know there is no such adaptor.
You can get a USB to PS2 adaptor to allow you to plug your keyboard into the PS2 port on the back (If the Desktop has one).
You dont mention how you diabled the USB, if its in the bios you can just re enable them, if its in windows, then you may be able to re enable them in safe mode.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix desktop pcs.