I was trying to upgrade a SQL 2000 server to SQL 2005 and the upgrade kept failing.  The first error I would see was "An installation package for the product Microsoft SQL Server Native Client cannot be found. Try the installation again using a valid copy of the installation package 'sqlncli.msi'."  Other parts also failed to install.  I looked at the installation log files, but they were so large and there were so many messages about it trying to locate packages, registry entries, etc., it was impossible to tell what represented an error and what did not. [more]

I opened a case with Microsoft (which could be a pretty good gotcha in itself), and sent the log files.  The tech seemed to know immediately what the problem was.  It was caused by the fact that a Microsoft SQL Server Native Client was already installed on the machine.  I think it was installed by the some third party software and it was a later version than what came with SQL 2005.  I removed the Microsoft SQL Server Native Client and the installation went fine with no errors.  Later I received a follow up call from Microsoft and they said they had received many reports of this issue and they would have this fixed in SQL 2008.