If you've ever had Exchange 2007 setup fail partially, you've probably noticed that some of the components (maybe even the one it failed during) show up as installed, but, if you run the ExBPA (Best Practices Analyzer), it will probably complain that setup failed. With most applications, you can open the Add/Remove Programs (or Programs and Features in WS2008) Control Panel and begin a repair. Unfortunately Exchange does not offer a repair option.
The trick with Exchange is to fix it with the exsetup utility. If, for example, you need to repair a Mailbox server role installation, go to the Exchange installation folder and run "exsetup /role:Mailbox". The utility will resume setup where it left off and clean things up.