Sorry, I made typo in my previous post.

There are no instances nodes under HKLM\Software\Microsoft\Microsoft SQL Server.

However, I did find it under:

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Microsoft SQL Server\MSSQL.1

It would appear that the 32bit version of SQL2005 was installed on the 64bit OS, which is why it got confused.

Will have to check which media was used for the install, because I thought it was just the standard MSDN DVD installer which should hopefully have installed the right edition.