The term “Rollback” refers to an operation that returns the Exchange server to its pre-installation state. “A rollback mechanism was added to Exchange 2007 SP2, Update Rollup1 to resolve this problem —”Cancel” button is now enabled to let you cancel installation during most of the process. If cancelled, installer will now rollback any changes, including starting stopped Exchange services. We still prevent cancellation of installation once it reaches post-install NGen step because it makes no difference in terms of amount of work involved; we still need to finish NGen for new and updated files and then run NGen again on old binaries once rollback has completed. You’ll see confirmation dialog for canceling if “Cancel” button was clicked. However, if rollback is due to an installation error, no confirmation dialog will show up. Any programmatic cancel action will however be recorded in installation log file along with error. The time it takes to run rollback procedure depends how far installation has advanced when the process is interrupted. The important thing to know about rollback is that system will be restored to the state before installation was started,” Microsoft expains.