Using Multiple Oracle Homes on Windows
This configuration is supported but you should be careful to observe the following precautions: 

1. Each Oracle product should be installed in its own ORACLE_HOME. In other words, each product should be installed into a different directory structure than other Oracle products. 

2. Ensure that the \ORACLE_HOME\bin directories for each product are listed in the correct order in your PATH environment variable. If you do not, there is a possibility that Windows will load the wrong .dll file(s) for some Oracle tools or 3rd party products installed in an ORACLE_HOME. 

Oracle itself and most of the tools that come with the Database will load and run correctly, but 
if you try to run some of the Oracle9i tools (for example, the DBCA utility), Windows might report an error because it loaded files from the Oracle 10g ORACLE_HOME and not the 9i ORACLE_HOME as it should have done. 

To resolve this situation, you must set your default ORACLE_HOME\bin in your PATH environment variable to be the same as the bin directory for the tools you are going to run. 

To change the default ORACLE_HOME, you need to: 

1. Run the Oracle Universal Installer (OUI), version 10.x or higher 
2. Click on the Installed Products button 
3. Select the Environment Tab at the top of the form 
4. Move the ORACLE_HOME that you want as your default, to the top of the list. 
5. Apply the changes, and exit the OUI. 

or you can set the PATH environment variable by issuing:
setx PATH "c:\oracle\dbhome2;%PATH%;" /M