So how do I install this INF file?
Basically you need to re-install your modem. Most of the companies provide instructions on their website or in a ReadMe file. This is useful as they use different methods of file compression. But the re-installation process is essentially the same.
Remember which folder you have put the INF file in. (Some companies eg Pace, and my own file, automatically install into the C:/Windows/Inf folder, this makes things slightly simpler). Then run "add/remove hardware" from the Control Panel, DON'T allow Windows to detect it, select Modem, and check "Don't detect my modem I will select it from a list". Unless the INF file was automatically installed as above, you should click "Have disc..." when the list is presented, and browse to find the folder you put the INF files in. Click OK and it will reinstall your modem. It may or may not ask you about new hardware during this process, again click "Have disc" if it does (usually the sound files for a voice modem). Once it has all been installed, you will need to select the modem in the properties of your Dial Up Networking connection, (not forgetting to configure it) and add the init string as detailed above. Note no string is required with my modeminf.exe as it is inbuilt.