MS SQL Server 2005 SP4 Installation failure // Error #1406

While performing SQL Server 2005 SP4 patching, I had recently faced the following error message and it immediately
prompted me to verify the permissions (Account already had full privileges on data/system drives) granted to the account used for this patching purpose.

Log File                  : C:\Program Files\Microsoft SQL Server\90\Setup Bootstrap\LOG\Hotfix\SQL9_Hotfix_KB2463332_sqlrun_sql.msp.log
Error Number              : 1406
Error Description         : MSP Error: 1406  Could not write value  to key \Software\Classes\.pre.   Verify that you have sufficient access to that key, or contact your support personnel

While I had ensured that all of the below per-requisites were covered prior to the start of this patching activity, I continued to receive errors (Could not write value  to key \….\.   Verify that you have sufficient access to that key, or contact your support personnel)–
1) Presence of Built-in Administrator in SQL Server.
2) Complete access on windows file directory, where system DB’s are hosted.
3) Administrator privileges on Windows account.

In order to set the registry key permission to allow full control, I followed this-
1.  On the Windows Task bar, click Start…Run.
2.  Type “regedit” (without the quotes) and click OK.
3.  Browse to the specified registry key.
4.  Right click the key and select “Permissions”.
5.  In the Group or User names field, select the user installing the program.  Be sure the user has administrative rights.
6.  Under the Permissions field, make sure “Full Control” and “Read” are both set to “Allow”.
7.  Click OK and attempt the operation again.

It was all in vain. So, what was preventing it from progressing?
Answer was right in front of my screen:  Anti-Virus 🙂

I tried to disable the first 3 items on the screen below and then disabled the “On-Access Scanner”.
Started the installation process once again, and boom… It went absolutely fine and finally, the server is now patched with 2005 SP4.

The problem was the anti-virus was blocking the process of writing into the classes.
Disabling it made my work done in a swift manner!

Hope this helps.

Cheers!
Aman Kharbanda