Ok, first need to know if you are using a USB connection or ethernet. If it's USB then it is plausable that yourr modem itself needs to be installed.. if you don't have the cd that came with the modem you will need to talk to your Internet Service PRovider and ask for a new copy.
If your connection is ethernet.. are you running windows XP? If so, then it is likely that you can just uninstal your network card from device manager and reboot your computer.. generally on reboot windows xp can instal from its own directory of drivers.
Failing that you will need to contact either your computers manufacturer or the retail business that sold you the machine to get it installed.
I hope this was of use to you.. In the future if you have problems I would recommend visiting
www.isitworking.co.nr
They seem to have a fair bunch of stuff available for internet troubleshooting and such.