Websphere installation is corrupted?
Not able to perform proper uninstallation ? your WEBSPHERE directory got deleted ?
Not able to install Websphere into default locations ? want to do installation on clean machine ?

You can reinstall without a clean machine. Such an installation creates a coexistence scenario that can prevent you from installing into the original directory.

• Log on as the same user ID who installed the product.
• Kill all the Websphere related processes especially stop java processes running due to Webshere.
• Run the uninstaller which deletes WAS stuff    /opt/IBM/WebSphere/AppServer/uninstall/uninstall

      If you are not able to run the above step also not to worry,continue the next steps

• Next step is to remove WAS relates packages,
• rpm -qa | grep WS
• Using the above command identify the packages to be removed

      Version 6.1 package names have a prefix of WSB or WSP and a suffix of 61. WSC package names do not have a suffix of 61

• From the above list remove unwanted packages using below command.
• rpm -e
• Now remove directory structure using rm -rf /opt/IBM/WebSphere/AppServer
• Edit the "vpd.properties" file to remove WAS related entires. You can find this file under the home directory of the user who installed the WAS
• Use the installRegistryUtils command to examine the installation locations for all installed WebSphere Application Server products and remove the desired products from the install registry.
• You can find installRegistryUtils in two locations
• one is in the WAS installer/build CD or images location   -    .... WAS/installRegistryUtils/bin/installRegistryUtils.sh
• If your installation still available it will be in /opt/IBM/WebSphere/AppServer/bin/installRegistryUtils.sh
• To clean up all accessible products and packages.
• ./installRegistryUtils.sh -cleanAll [-userHome user_home]
• ./installRegistryUtils.sh -listProducts [-userHome user_home]
• ./installRegistryUtils.sh -listPackages [-userHome user_home]

Now you are all set to install the WAS in the default locations as clean install.

Note : The above steps are in-view of Linux or other Unix flavors. Slight understanding of folder structure in windows help you to do the same in windows

Reference : [1][2][ AIX: 3]