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. Most applications you just go to Add/Remove Programs (or Programs and Features in WS2008) and there’s a repair option. Exchange of course has no such option.
The trick with Exchange is to fix it with the exsetup utility. If for example you needed to fix up a Mailbox server role installation, you’d go to C:\Exchange\Bin (or where ever you installed Exchange to) and run "exsetup /role:Mailbox". The utility will resume setup where it left off and clean things up.