Manually download and install the SP2 :-
http://www.microsoft.com/en-au/download/details.aspx?id=39667 (32 bit)
http://www.microsoft.com/en-au/download/details.aspx?id=39647 (64 bit)
If unsure of whether or not you're running 32 bit or 64 bit, refer to this thread:
If you still have trouble, try updating in a clean boot state:
http://support.microsoft.com/kb/929135/en-au (don't forget to return to normal boot when finished).
Office updates can fail if the update files are looking for the original installation files.
Sometimes downloading the full file remedies that problem or putting the installation media in the installation drive can also assist.
If still failing, perhaps you'll need to perform a repair install.