There was an unexpected failure during the setup wizard

OK folks, this is a little bit "tricky" issue.

The scenario is the following:
You are trying to install SQL Server 2005, then you stop the installation, let's say because you got a warning on a prerequisite that you would like to fix before proceeding. However, in the case you abort the installation, fix the problem and try to install SQL Server again you now get the following error message:

There was an unexpected failure during the setup wizard. You may review the setup logs and/or click the help button for more information.

So, what do you do?

As I've faced this issue a few days ago, this is the solution I have concluded to:

1. Delete Everything in the folder "C:\Program Files\Microsoft SQL Server\90\Setup Bootstrap"
2. Enter the registry (Start--Run--Regedit)
3. Delete all the SQL Server 2005 "Setup Bootstrap" keys in "HKEY_CLASSES_ROOT\Installer\Assemblies". Example: "HKEY_CLASSES_ROOT\Installer\Assemblies\C:|Program Files (x86)|Microsoft SQL Server|90|Setup Bootstrap|BPA|bin|BPAClient.dll"
4. Restart the machine
5. Start again the installation of SQL Server 2005

The philosophy behind this solution is that you actually need to "clean" the machine from any "pending" installation setup settings/files/registry keys/etc. of SQL Server 2005 before you attempt to start a new installation.

The reason you need to do this is because when you start a new installation, if it finds remains of a previous installation it "reads" it and proceeds based on those.

*** Last but definitely not least:
Be extremely careful when you are modifying the Windows Registry. Always take a backup before you do anything and of course always remember what's written on Registry's door: Enter at your own risk! :)

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