You should have no problem finding a PCI Express card with serial ports on it, as well as normal PCI. Then you could plug an external modem into it. If you don't get your USR modem doing what it's supposed to and end up trying another modem, your next modem for reliability's sake should be an external serial modem.
Modems that connect to a legacy serial port tend to be of a vastly superior quality than ones that connect to a PCI slot or USB port. The price reflects that. That's because PCI and USB allow manufacturers to take horrendous shortcuts designing your modem by making your CPU "emulate" the modem's most expensive parts. See "Winmodem" on Wikipedia for an explanation.