This generally happens, when one of the following files are modified, either by the user, or by some open source software being installed.
Chances are you have one of the following files and it has the bad PATH information
.profile
.bash_profile
.bash_login
.bashrc
Most of the above do not exist, but I strongly suspect one of them does.
The easiest thing to do is rename the offending file, then start a new Terminal session.
Terminal -> Shell -> New Command -> ls -a
This should display the contents of your home folder, including all the files that start with a period, such as the .profile, .bash_profile, .bash_login, or .bashrc
Once you know the name of the offending bash shell initialization script, you can rename it using:
Terminal -> Shell -> New Command -> mv .profile save.profile
Terminal -> Shell -> New Command -> mv .bash_profile save.bash_profile
Terminal -> Shell -> New Command -> mv .bash_login save.bash_login
Terminal -> Shell -> New Command -> mv .profile save.bashrc
Based on the previous ls -a command you should know which of the 'mv' commands you need to rename the offending shell initialization script.
Another approach is to download TextWrangler
<http://www.barebones.com/products/textwrangler/>
which is free, and can safely edit shell text files, plus it will actually display hidden files, such as the ones that start with a period.
Once you have safely moved or fixed the offending shell initialization script, you should be able to start a new Terminal session and have a clean PATH.
Now you can look at the old initialization script and fix it, knowing if you make any mistakes, you can use the above to get back to the beginning and try again.