Office File Validation, or OFV, was a new security feature (an add-in) released in Office 2010 and is installed as part of MS11-021, released in April 2011. It was back ported to Office 2007 and 2003. KB Article 2501584 states, "Office File Validation verifies that a particular binary file complies with the application’s expectations. Office File Validation can help prevent unknown binary file format attacks against Microsoft Office 97-2003 file formats. " (http://support.microsoft.com/kb/2501584).

However we have seen situations where, after applying MS11-021, opening Office files is significantly slower from a network location. KB Article 2570623 (http://support.microsoft.com/kb/2570623) was written for this purpose. The problem typically only affects Excel 2003, however it can affect Excel 2007 and 2010 if you have ever set the 'DisableRobustifiedUNC' registry key. If you're using Excel 2003 and are seeing an increase in time taken to open Excel files from a network location, like 40 seconds to 5 minutes, then you will want to check out KB article 2570623 (http://support.microsoft.com/kb/2570623). If you are on Office 2007 or 2010 and see an increase in time to open Excel files across the network and you can confirm that MS11-021 was recently installed, then check for the following registry key:

For Excel 2007

HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Excel\Options

For Excel 2010

HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Excel\Options

Look for a DWORD value labeled DisableRobustifiedUNC set to a value of 1. If this is set on your machine, it's recommended you delete this value or set the DWORD value to 0.