Your Browscap.ini file is probably outdated, or you are using an obscure browser. Unfortunately, developers are pretty much left on their own these days when it comes to Browscap.ini. See
Article #2199 for information on obtaining a more recent copy of Browscap.ini.
If you want to avoid using Browscap, see some tips for manually detecting browsers in
Article #2184.
In addition, CyScape makes a great browser detection component called
BrowserHawk -- reducing the need for much of the manual coding to provide proper browser detection.