I can't find drivers for the PC/X product in Microsoft Windows.

Solution:

**Digi International has decided the PC/X, PC/4, PC/8 or PC/16 will not be a supported product for it’s Tech Support Division for customers using Microsoft software for the following reasons:

This product is a non-intelligent device with no device drivers available.  In the past, Digi has assisted with registry input and correction., Digi will no longer assist in registry input or correction for non driver supported boards.  Digi International Tech Support will not walk the customer through a configuration of the product -- this is covered by current documents located on the web page.  Digi International will assist in running Digi International diagnostics with this product to confirm that the board is functioning properly. The diagnostics program is not required to run unless there is a problem with the installation of the device.  The most common problem is a resource conflict. 
NOTE: Sometimes it simply requires installing one port at a time.

Installation requires that you first follow the manual to set up the dip switches (I/O) and jumpers (IRQ).
http://ftp.digi.com/support/manuals/hardware/pcx/90029700A.pdf

Then follow the operating system instructions:
1. Windows 9x: ftp://ftp.digi.com/support/drivers/win98/pcx_win98.txt
2. Windows NT: ftp://ftp.digi.com/support/drivers/winnt/pcx_nt.txt
3. Windows 2000/XP: http://ftp.digi.com/support/techsupport/nonunix/win2000/classic_isa/index.html