It can slow it down. Because of that, WinModems do not work under DOS or when running other operating systems like Linux. For just a little more money, you can get a hardware based modem which is what we would recommend. Plus, they work with any type of operating system. Now sometimes it hard to tell the difference between a WinModem and a standard modem.
All external modems are hardware based, so that one is easy.
With the internal ones, it can get a bit tricky. If you are shopping for a modem or are purchasing a new system with a modem installed, make sure that you know what you are getting.