Causes of the Error

• The Bootmgr compression error can occur in two situations. The most common is a corrupted install. Several things can cause an installation to corrupt, including unintended shutdowns and problems with the drive where the install disc is running. The second scenario that might trigger a compression error is when you have a dual-booted other OS that has updated and encroached on Windows Vista's allotted space. No matter what has caused the error, the fix will be the same.
  
  

Preparing to Fix the Error

• If you encountered the error when Installing Windows Vista on its own for the first time, you do not need to prepare anything before you fix the error. However, if you encounter the error while using a dual-boot system, try using your other OS to access important files you have saved on the Vista OS. The repair installation will completely wipe out any content you already have saved with the OS. Once you have saved the files you need, grab your Vista installation CD, as it is time to begin.
  
  

Performing the Repair Installation

• To fix the compression issue, insert the Windows Vista Ultimate installation CD and then select "Repair installation" from the menu. The system will then install the files that have been corrupted and restore your operating system to its original settings. The process can take up to 2 hours, so make sure you have some time cleared while the operation is taking place. Once the operation has finished, your computer will restart automatically, and Windows Vista will boot as normal.
  
  

Using Windows Recovery Environment

• If you still encounter this issue after performing the repair installation, you will need to enter the Windows Recovery Environment by selecting "Command Prompt" when you are prompted to select what type of installation you would like to perform during the installation process. Once the command prompt appears, type these commands exactly as they appear (without quotes): "compact /U C:\bootmgr" followed by "compact /U C:\ntldr." This should manually fix the problem, and the repair installation should continue.