Alternatively: Run Belarc Advisor, obtain your Microsoft Office Product Key. Uninstall any Microsoft Office Programs. Download the correct (x64) version from here (link to this and other Microsoft Software are available in the Microsoft Softwarewiki below aswell). Run installer, enter your product key when prompted and activate. Note Mathtype or Mendeley Desktop don't quite work in the 64 bit version of Office.
As many may have realized the Office 2010 installer shipping with Dell systems (even on 64-bit OS's) will only install Office 32 bit. To install Office 64 bit on a compatible 64-Bit OS please follow the following steps.  1. Ensure Office 64-bit is what you need, there are some limitations, review them here: [URL]#BKMK_Disadvantages 2. Uninstall all versions of Microsoft office and make sure none are listed in Programs and Features. 3. Restart and go to [URL] 4. Sign into or create a new Windows Live account.  5. Put in your Product Key, select Language and enter the correct Captcha.  6. Confirm order details. (You can order a backup DVD this way as well) 7. (Do NOT click on Download) Select the Advanced Options that is listed below the Download link.  8. Select 64-Bit to begin the download for the 64-bit installer. WARNING: Office 2010 64-bit is NOT compatible with WIndows XP 64.    
How to Download Office 2010 64 Bit