If I say Oracle is the best database system available then there will be hardly any one who will disagree with me. Oracle 8 / 8i were the best database systems and the biggest bug in Oracle 8 is, there is no uninstaller and after you install the Oracle 8 personal you don’t have an option of removing and without removing it you cannot install the later version or even upgrade it. So now the question arises how to uninstall the Oracle 8. The last option that came to my mind is to format the hard drive and so the Oracle 8 personal will be gone and as I wanted to format it started taking the backup. After all backup done being a programmer and database designer thought of giving a try as to edit the registry and remove it manually and lastly succeeded in the process. I am sharing my personal opinion how I removed it and following this steps can be dangerous and it’s recommended that whatever you do you should think it’s ok and not unethical. If anything unintended happens I am not responsible for the effect though. Say you have installed the Oracle at C:\ORANT\ Now the first step is to physically delete the directory using the Shift + Del Keys. After doing that open the registry by typing the regedit in the Run Command dialogue box. Start searching for the entries C:\ORANT and as you find them start deleting them but keep in mind that you delete the sole entries of C:\ORANT\ and subfolders entries and not the path variables and other variables where C:\ORANT is just a part of it. Manually edit the path variables. Also start for searching the folder in the left panel Name ‘ORACLE’ without quotes and delete that folder completely from the registry. After doing this Reboot the machine and now you are ready to install any Oracle version.
But there is a threat tht we may delete any other registry entry so its better to use any software tool such as 'Registry Cleaner' for removing registry logs. But doing this dont forget to take backup bcoz prevention is more better than cure.....
Doing manually there are less chances that you may delete something that is unncessary if you keep in mind