Has another Apache been installed? From a terminal (Applications-> Utilities-> Terminal.app) try:
ps ax | grep httpd
You should see something like the following if your local web sharing is enabled:
25829 ?? Ss 0:00.04 /usr/sbin/httpd -D FOREGROUND -D WEBSHARING_ON
25831 ?? S 0:00.00 /usr/sbin/httpd -D FOREGROUND -D WEBSHARING_ON
ps ax | grep apache
This should return nothing if you are using the default install, but might if another server was installed.
If apache is running from anything other than /usr/sbin, then you may want to investigate your PATH environment variable in case it has been altered.
You could also try using the default httpd.conf file supplied by Apple (in case yours has been edited).
Make a backup of the current one
sudo cp /etc/apache2/httpd.conf /etc/apache2/httpd.conf.bak
Copy the 'original' httpd.conf file
sudo cp /etc/apache2/original/httpd.conf /etc/apache2/httpd.conf
Restart the web server
sudo apachectl restart
On a side note.. If you are or will be doing any long term web development, I would advise installing apache2, php, mysql, etc from MacPorts or Homebrew. There is a little bit of a learning curve, but it's easy when you get the hang of it and you will benefit over the long haul. Example port commands:
Search for applications to install
port search apache2
sudo port install apache2 (currently version 2.2.22)
sudo port uninstall apache2
sudo port upgrade apache2
Also, MacPorts installs everything into the /opt/ directory (I think Homebrew does as well), so you can always just delete the entire directory if you do not want them anymore. Again, if you do not mind digging in the trenches a bit and can be patient while your computer compiles these apps from source code (this can be lengthy), it will be worth your while.