If you are running Excel 2003

OAK needs Version 2.0 of Microsoft's .NET framework in order function.  Excel 2003 expects to work with .NET 1.1, since that was the version that was current when it was released.

The .NET Framework 2.0 includes a lock-back policy that prevents Excel 2000-2003 from loading .NET 2.0 or later. This is easily disabled for Excel 2000 and 2002/XP, but without a particular update, Excel 2003 will still not be able to load .NET 2 add-ins.

This update must be installed in Office 2003 if OAK is to work properly.  Add-ins that have failed to load for this reason are put into the disabled items list, accessible from Excel's about box.

The OAK installer will remove the lockback policy. The minimal installer will notify the user of the problem if it finds it, the full installer will attempt to install the update. If the security on your computer is set very high in conformance with corporate policies, it may be necessary to make the adjustment manually.

At the end of this page are external links for reference. One of these makes various statements concerning the safety of running managed add-ins not made using Visual Studio Tools for Office (VSTO) in Microsoft Office. While OAK was not made using VSTO, its managed components are isolated from other add-ins by a layer of unmanaged (native) code.

See also

A Microsoft knowledgebase article explaining this point is at http://support.microsoft.com/kb/948461

A longer article on the same subject is at http://blogs.msdn.com/vsofficedeveloper/pages/Office-CLR2-Lockback-Bypass.aspx

The necessary update to Office 2003 is at http://support.microsoft.com/kb/907417/