Then it depends what OS your running, probably Win98 I guess. What you want to do is, right-click on your "My Computer" icon on the Windows Desktop, and then left-click "Properties". You then want to navigate in the pop-up window that appears, to something that's called "Device Manager", it might be under the heading hardware. Now if it's anything like mine, you should see a tree with a category called modems, if you click on that, it will expand (the plus turns into a minus) and there will be your software driver whatever your modem is called. You then probably want to right-click and select "uninstall" or "delete", and it will remove the software driver for you.
But obviously only do this just before removing the old modem and installing the new one!
Good luck.
Sharky.