Uninstall 11g from Windows

In 11.2g there is deinstall command through which you can uninstall standalone Oracle Database installations, Oracle Clusterware and Automatic Storage Management (ASM) from your server, as well as Oracle Real Application Clusters (Oracle RAC) and Oracle Database client installations.
You can find out deinstall tool under $ORACLE_HOME/deinstall folder.

You can also download it from :
http://www.oracle.com/technology/software/products/database/index.html
(Under Oracle Database 11g Release 2, click See All for the respective platform for which you want to download the Deinstallation Tool.)

Here are the steps you need to follow for Deinstalling Oracle 11g on Windows

1- Unzip the Deinstall Toll file set just downloaded and copy it over to any directory you want
  or go to the Folder C:\app\product\11.2.0\dbhome_1\deinstall

2- Open command prompt(Run as Administrator) and go to the proper location

3- Set you ORACLE_HOME to the Home which you want to deinstall using this command
set ORACLE_HOME = C:\app\product\11.2.0\dbhome_1

4- Now you are ready to unistall it. Tool is being run outside the Oracle Home
Type:
C:\app\product\11.2.0\dbhome_1\deinstall\deinstall -home C:\app\product\11.2.0\dbhome_1
At this point Oracle will ask you several questions and it will start the deinstallation process.