The link I made was actually to the Behringer USB interface which could work for a keyboard that has audio out capabilities, but I was thinking of it more in terms of other instruments, such as harp, guitar, etc. With one of these it could be used with a pickup attached to the instrument then run through a preamp to provide some line level output to the inputs on the Behringer.

For a keyboard, the midi/usb cable linked to by M. Ted in the third post of this thread should do the trick, even for your old keyboard. The keyboard I used with this type of cable is an old MusicStar which was made for Win 3.1, after which the company promptly went out of business and no drivers were ever created for any Win versions above 3.1. It still worked fine for me on computers using both Win XP and Vista (using native Windows drivers).