Rockfire Taiwan obtainable at Radio Shack or "Maplins" in the UK make a USB to gameport adaptor. However, on XP OS if your CH joystick/and/or throttle is a programmable one you won't be able to program it with Speedkeys in XP, it will only work in Win98/ME, unless you have a a double boot system for both OS's, once programmed in 98 it will still work in XP via a gameport.
You will only be able to use the Windows CH stick defaults in XP via the Rockfire USB adaptor, but it will work with AHs own joystick and keymapping as a 4 button stick I believe so it is still usable. Depends what CH stick you have. You "could" fit a PCI soundcard with a gameport to your new PC, and disable any onboard sound, to still use all functions on CH programmble sticks, but sound card gameports on newer fast PCs tend to spike unless the new MBs have an onboard gameport.