Determine sql server installation (server or client tools only)
take a look here :


  HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSSQLServer 
  HKEY_LOCAL_MACHINE\Software\Microsoft\Microsoft SQL Server


SQL Server 2008 R2


HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\SQLNCLI10\CurrentVersionSQL 


SQL Server 2008


SQL Server Native Client 10 HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\SQLNCLI10\CurrentVersion 


SQL Server 2005 


SQL Server Native Client 9 HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Native Client\CurrentVersion