AumJa24 --

Error : orainst.exe has encountered a problem and needs to close. We are sorry for the inconvenience.

To solve this error, you must stop Oracle's services and rename its installation folder.
After that, backup its registry key (HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE) and then remove it.

Install Developer 6i, restore registry backup and rename Oracle's folder to its original name.