Here are the steps to follow to uninstall a clustered Exchange 2003 in a correct way as simply as possible:
1) Go to the passive node and from add / remove programs remove exchange 2003, when you will be asked if it is the last node you have to answer NO. N.B. You'll need the CD / ISO to uninstall the software.
2) Go on the active node and from the management console of the failover cluster put offline “Exchange System attendant” (EVS System Attendant)
3) Remove the Exchange virtual server from the cluster groups
4) From add / remove programms remove Exchange 2003 and when you will be asked if it is the last node you have to answer YES.