i've been having the same issue and it was driving me crazy.
I'm guessing you have installed and unistalled it before and now you are installing it again. Before it give you the error it says its found a previous version has been found at . and its going to upgrade.
The reason it does this is because it thinks that apache/php is still installed.
this is the fix for windows server 2007, but should apply to other windows installs.
The fix is:
Go to start, and type regedit
expand the folder HKEY_LOCAL_MACHINE
expand the folder SOFTWARE
expand the folder ENTERPRISEDB
then there should be a folder there, not sure what it was named but probably apache/php
right click this folder and delete it. then try to reinstall again.
I had a problem installing Apache/PHP v 2.2.20 with Stack Builder 3.1.0 on XP service pack 3
The Stack Builder wizard erroneously found a pre existing Apache server after it had been uninstalled.
I did sinisterdeath's registry hack and the install wizard worked.
Stack Builder does not state that Apache/PHP v 2.2.20 is installed.
Although it does report that postgres and phppgadmin are installed. This can be confusing.