I am using Oracle 9i on a Windows 2003 server. Unfortunately the server went down and I didn't have a backup; therefore I had to re-install the OS again. Is there any way to re-attach the existing database with only Oracle software installed on the same system?
Without a backup, chances are good that you will not be able to recover your database. If you do have your database datafiles, you can try to copy them off, recreate a shell of a database and then replace the shell db's files (along with control files and online redo logs) with the ones from your original database. Depending on a number of factors, you may or may not be able to get the database up and running.
A good backup and recovery strategy is essential. Once you have worked past this issue, please implement a sound backup and recovery strategy for all of your databases.