When using Office 2007 a customer receives an error message when exporting XForm posted data to excel. "The file you are trying to open, '[filename]', is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now?"
According to http://www.grinn.net/blog/dev/2008/06/file-you-are-trying-to-open-is-in.html this is a "feature" of Excel 2007. However, the customer does not experience this with their EPiServer 4.61 site, only in CMS5 R2. Is that just a fluke or can I do something to prevent the warning?
If your customer uses Vista he can get some problems if he installs the Client Component with UAC turn off. The component will then be installed by a user who is not in the context of being an administrator. The component will actually not be installed even if it looks that way. The solution is to turn on UAC, boot machine, install component, turn off uac, boot machine. (Maybe even just rightclick the component and choose "run as administrator")
I don't know if this is your problem, but it is worth checking.
I checked with the customer and they are using Windows XP / Office 2007, so UAC is not the issue, I'm sorry to say. But could it somehow be the episerver office integration?