If your old stick still works fine and you wish to keep using it try a converter.
http://www.cableguyusa.com/cgi-bin/cable/601-733.html
Here's the problem:
"Supports conventional analog game controllers."
"* This converter is for Analog Joysticks and Gamepads ONLY, it will NOT work with Digital joysticks"
The Sidewinders were all digital sticks (the only digital sticks built to run through a standard gameport). There's something (that I don't really understand) in the way they utilized the 15 pin game port to carry the digital signal to the proccessor that isn't normally built into a 15 pin gameport to usb adapter. These are all built for 4 button analog joysticks.
To add to the confusion, the few adapters out there that claim to work with Sidewinders typically only work with select models. I've found converters for the Precision Pro and for the Force Feedback (both of which originally were available with a usb adapter), but not for a 3D Pro which was the first Sidewinder joystick.
I did find another gameing site where some guy was building converters for 3D Pros on circuit boards but it sounded like it was only very rarely and by request when there were enough requests to warrent buying the boards plus it was more or less a "I'll send you the parts, you build it" kind of thing. I might drop a note on that site and hope one day I get one. Alternately, I could always dump my Creative X-Fi Gamer and get a cheapie soundcard with a gameport I guess but the X-Fi has it's own proccessor and RAM which is nice.