You can use these docs for uninstalling your 2005 sql instance based on your requirement.
Uninstalling an instance of SQL Server 2005 manuallyhttp://support.microsoft.com/?kbid=909967
Windows Installer Clean-Up Toolhttp://download.microsoft.com/download/E/9/D/E9D80355-7AB4-45B8-80E8-983A48D5E1BD/msicuu2.exe
Uninstall SQL Server Management Studiohttp://support.microsoft.com/default.aspx?scid=kb;EN-US;909953
Using the Add or Remove Programs item in Control Panel to add or remove components for stand-alone installations and clustered installations of SQL Server 2005 (KB: 922670)http://support.microsoft.com/default.aspx?scid=kb;EN-US;922670
Uninstall SQL Server Express http://msdn2.microsoft.com/en-us/library/ms143505.aspx
Fatal error during installationhttp://support.microsoft.com/?kbid=919945
Uninstall Applications NOT in Install/Remove Programs Listhttp://blogs.msdn.com/astebner/archive/2005/10/30/487096.aspx