I'm going nuts here...

I've got 4 NT Workstation systems that will not let me install any modem. The customer wants USR, and all we have are the 5610's (Performance Pro(.

Specs. (all the systems are the same)
Dual Intel P3 800/133
512 ECC SDRAM
3d Labs Oxygen VX1 32m
20g WD HDD
3com 905c 10/100 NIC
400w SuperMicro PS
SuperMicro FullTower Case

I've instaled PNPISA.INF, installed the latest drivers, reinstalled SP6. I even put a Diamond Supra in one, and it didn't work either.

What am I doing wrong/missing/forgetting?