SQL Server 2005 Reporting Services Option grayed on setup screen

Hi, 

i Need to get Reporting Server installed on a SQL 2005 Standard x64 Server with SP2.

When i get to the instalation screen the Reporting server option is grayed out.

i Also get this version:

- Edition Change Check (Warning)

Messages

·Edition Change Check  

To change an existing instance of Microsoft SQL Server 2005 to a different edition of SQL Server 2005, you must run SQL Server 2005 Setup from the command prompt and include the SKUUPGRADE=1 parameter.

Is this due to service packs that is installed?

i Tried changing the app pool default from true to false in IIS but no luck,

How do i get around this?

How do i run the  SKUUPGRADE=1 if i only want to add reporting services or is there another way?