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Here's the Step-by-step Procedure to Install Exchange Server 2007 SP3 in you Windows Server 2003 64bit R2.

• First action is to download the Exchange Server 2007 SP3, here's the link where to download Exchange Server SP3 http://www.microsoft.com/downloads/en/default.aspx or just search "Exchange Server 2007 Service Pack 3" in any search engine.
• Save the file to your Windows Server temp file. Before you can execute this file in Windows Server 2003 environment, you need to UNBLOCK by right-clicking on file. Note: If you don't UNBLOCK the file, you will receive this similar error when you double-click "Windows cannot access the specified device, path, or file. You may not have the appropriate permissions to access the item."
• You will also need to install the Windows Installer 4.5. After the installation of Windows Installer 4.5 restart the server.
• Let's start the installation of Exchange Server 2007 SP3. Click on Setup.exe file.
• In Exchange Server 2007 SP3 Setup, click on Next to continue.
• In License Agreement, click "I accept the terms in the license agreement.", then click Next. The installer will check the readiness of your Exchange Server. An error "You must be a member of the Exchange Organization Administrators group to run the upgrade." To resolve this error, in your Active Directory Domain Controller add you "Administrator" account in the Exchange Organization Administrators group. You can see this group in Microsoft Exchange Security Group OU.
• Log off, login, and re-run the Exchange Server 2007 SP3 installer again.
• Another Error "Setup cannot continue with the upgrade because the "beremote" {}process (ID 2628) has open file. Close the process and restart Setup." 
• This is easy to correct. In your Services, stop the Backup Exec Remote Agent for Windows Server". Then in Exchange Server 2007 SP3 click Retry to continue.
• Click Upgrade. This will start the upgrade to Exchange Server 2007 SP3 and will show you the progress of installation from Organization Preparation, Preparing Setup, Remove Exchange Files, Preparing files, Copy Exchange Files, Hub Transport Role, Client Access Role, Mailbox Role, Management Tools and Finalizing Setup. At this time, you may take a break. This upgrade will take approximately 40 minutes or sooner.
• Click Finish to complete the upgrade.

Testing. 

Login to your MS Outlook or Outlook Web Access (OWA) to send and receive emails. If this successful, that's conclude your Exchange Server 2007 SP3 Upgrade.

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