I would take a look here first:

How to: Add or Remove Nodes in a SQL Server 2005 Failover Cluster (Setup)

https://msdn.microsoft.com/en-us/library/ms191545(v=sql.90).aspx

How to: Remove a SQL Server 2005 Failover Clustered Instance (Setup)

https://msdn.microsoft.com/en-us/library/ms190438(v=sql.90).aspx

(Hopefully this is not required, but just in case)

How to: Manually Uninstall a SQL Server 2005 Failover Cluster

https://msdn.microsoft.com/en-us/library/ms180973(v=sql.90).aspx