Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Try registering the windows installer as given in the article below:-
http://support.microsoft.com/kb/555175
Note:- the steps are similar to windows 7 also
You can also repair .net framework as suggested in the thread below:-
http://social.msdn.microsoft.com/Forums/en-US/netfxsetup/thread/87a83f8a-263c-40ff-b768-e371b0a2125e