What does Oracle XE perform during installation? I have oracle11g2 installed on my noteobook. 

I then installed the Oracle Database 11g XE. Now a .dll which uses the oracle11g2 client does not work anymore. First I recogniced that I had now two tnsnames.ora, which I resolved by linking the new one to the old one. But still my program cannot connect via the dll and throws an error.

I am relatively sure that the installation broke something which worked before so I need to know which steps the Oracle 11g XE installation procedure performs to system variable (like PATH) or to registry entries which might be important? Is there something like an installation log or can I somehow revert all changes which were done during installation? 

My system is running Windows 7 x64.

Best regards, Fox
XE changes the following:


tnsnames.ora
Path variables
takes the default port 1521 for itself and this cannot be changed


The best way to install XE and other products is to install XE first, then any other Oracle version afterwards.

I think you can fix this by 


examining the path variables by entering PATH at the command line
ensuring that each installation has a file path to the bin folder, something like this

c:\oracle\db11g\product\11.1.0\db_1\bin;
c:\XE\product\11.1.0\db_1\bin;
check that the listener.ora has the correct parameters by starting a command line and entering lsnrctl status  This will show you which listener is in use, it's location and what databases it is servicing.
change the port for the non XE database to anything other than 1521 in the listener.ora