I installed the latest version of gcc recently and I seem to be having problems compiling some programs since the old version of gcc is still there. It looks to me as if some programs are using the old headers but the new compiler...

The first thing I tried was reinstalling the new gcc, but now THAT wont compile, seemingly because of the same problem - i.e. whilst running make I see in the output /usr/include/c++/...../, but the new gcc is in /usr/local/gcc-3.4.2/....../ (cant remember the details of version, directory, etc right now).

Are there any config files or environment variables to set to tell linux which gcc to use? Should I remove the old gcc to remove the confusion?

Gentoo had a problem related to this a while back: I don't know if your distro supports it, but there was a script called fix_libtool_files.sh that resolved the issue.

What are the exact errors you are getting?