You can use these docs for uninstalling your 2005 sql instance based on your requirement.

Uninstalling an instance of SQL Server 2005 manually
http://support.microsoft.com/?kbid=909967

Windows Installer Clean-Up Tool
http://download.microsoft.com/download/E/9/D/E9D80355-7AB4-45B8-80E8-983A48D5E1BD/msicuu2.exe

Uninstall SQL Server Management Studio
http://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 installation
http://support.microsoft.com/?kbid=919945

Uninstall Applications NOT in Install/Remove Programs List
http://blogs.msdn.com/astebner/archive/2005/10/30/487096.aspx