First of all you can check the ADSL installation. You should either have a dual socket BT box on the wall. Your ADSL modem goes into the small (RJ11) socket and the telephone into the standard BT type socket. Alternatively you will have a standard BT socket into whch you plug a filter which itself has two sockets as described above. If you have extension sockets then ALL of them must have a filter into which you plug any extension phones. You could also ask BT to check that ADSL is working on your line. (BTW. They spent weeks arguing with me that it was and it wasn't.)
Having got that right if you still have a problem. I assume that your modem is attached via USB. Uninstall the modem driver and reinstall following the installation instructions TO THE LETTER. People often plug things in in the wrong sequence and some USB device take exception to it.
Terry
May 2006