Unlike UNIX oracle apps ,Windows bit different and tricky to clean the old instance . If your a Windows R12 oracle apps customer this post will be very useful . Please follow the note to delete the failed/old oracle instance .
How to Remove an Oracle E-Business Suite Release 12 Windows Environment (Metalink Note 567507.1)
More information available on Steven Chan blog – http://blogs.oracle.com/stevenChan/2008/07/new_whitepaper_cleaning_up_app.html “New Whitepaper: Cleaning Up Apps 12 on Windows“