|It now seems that it is Office 2007 that is trying to re-install, not just Excel.|
As both Excel (eg. xls) and Word (eg. doc) are causing this to happen, it sounds more likely that the whole package (Office 2007) that has gone wonky. Have you uninstalled Office 2007 and re-installed it? I appreciate that if Office 2007 came pre-loaded then that is not an option - only if you have an Office 2007 CD.
I am assuming that you have the full Office onboard and not just the MS viewers for these file types.
Always pop back and let us know the outcome - thanks