Upgrading from SQL 2000 to SQL 2005 in a clustered environment is very similar to going through the upgrade process in a non-clustered environment.
After verifying that your code is ready for backup, stop your clients from connecting, and backup the database (or backup the last log). If the upgrade fails badly enough you’ll need the backups to rollback.
Fire up the SQL 2005 installer. Tell it the services you wish to install making sure that you tell it to install SQL as a clustered install (it’s a checkbox on the service list screen). Make sure to tell the installer to install on on the same instance that you are currently using. This will cause the installer to upgrade the existing instance.
Run through the installer and when done install the latest service pack and hotfix.