A. Check the following :-
1. Make sure that there are no quotes in the path on the machine you are setting up.
2. Check that the full directory path you are installing from does not include any spaces.
3. Check the registry entry \[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSSQLServer\Setup\] ends in a "\" and add one if it doesn't.
4. Make sure that named-pipes is enabled and is the default net-lib on the server.
5. Check that SQL Server isn't starting in single-user mode
6. Make sure that the "sa" password does not contain blanks or any other special characters.
7. Make sure that the master database has at least 3MB free space.
Also make sure you are installing SP5a (SP5 was pulled for other reasons). SP5a is build 416. The "bad" SP5 was 415.