What chipset is on your motherboard? (VIA, SIS, INTEL, VX-PRO, ALi?)
If your motherboard had a CD-ROM that came with it, you can find the PCI Bridge software there. It's very specific to the Chipset on your motherboard. It may also have the USB supplement.
Here's a location to get the USB sup.
http://www.walbeehm.com/win95upd.html