oracle failsafe - how to start instance in nomount
Shutdown instance in Cluster Manager (Right click, "Take Offline" or however you choose to do it)
Bring up Windows Services (Start->Run->Services.MSC) on the clustered server
Start the Oracle service for the instance you want
Now you can do whatever you like in SQL*Plus (Cluster Manager will still show it as down no matter what you do at this point)


So, if step 3 brings the Oracle instance up to "Open" you can do a shutdown immediate etc and then open it nomount.

HTH