Welcome to the OptoForums! We do still sell/support that part (and even support it with more modern Operating systems!).
Check out this form, especially page 11: http://documents.opto22.com/1165_PC_Based_IO_Control_Systems.pdf
Also don't hesitate to contact our (free! friendly!) support team, email is always best: email@example.com
However, I am curious about what that test equipment consists of, and what you're up to in general. Normally that AC24AT card is used for communicating with custom software and our I/O brains. What is that card attached to? Did it come with some (Maccor?) software you're planning to install on the Windows 98 machine? (Maybe the software only runs on Windows 98?) Any/all details you can share will be helpful...