"Class not registered" error occurs when attempting to connect to a database or by opening a new Editor Window after upgrading to Toad for Oracle v9.0.
The QExplain2Full.msi installer may not have been executed during installation or there may have been files corrupted when installing on top of a previous installation.
1. With Toad for Oracle v9.0 closed, locate the QExplain2Full.msi installer in the Toad install directory.
2. Run the QExplain2Full.msi.
3. If there are issues during the run of the QExplain2Full.msi file, try executing it via command line such as the following:
msiexec /i "C:\Program Files\Quest Software\Toad for Oracle\QExplain2Full.msi"
Install Toad for Oracle v9.0 into a brand new directory.
Install Toad for Oracle v9.1 into a brand new directory.