The installer is supposed to automatically install OpenAL in your system, and somehow failing that I could think that you'd end up in such an issue. One thing that comes to my mind is that you'd possibly need to run the installer as an admin; any chance you're not logged in as an admin in Windows? You should be able to right-click on the installer and choose "Run as administrator..." to give it a go.
If that doesn't help, Google seems to have a few suggestions what to try next.
E.g.
http://support.frictionalgames.com/entry/78/ tells that you might want to try updating your audio drivers, or you might want to try manually installing OpenAL, it can be downloaded at
http://connect.creativelabs.com/openal/default.aspx, specifically at
http://connect.creativelabs.com/openal/ ... alinst.zip. Also a third solution is presented, which involves some manual cleaning up, check the first link.
Let me know if any of these help!