Well for some reason the Office Program has become corrupted. I would try doing a repair install of Office on the computer and see if that fixes your problem.
If it doesn't uninstall the entire application reboot and reinstall but do not remove any Shared Files when you uninstall or you may very well not be able to get into Windows again.
If that doesn't work you'll have to do some diagnostics to find out if you have a Hardware Problem and I would recommend The Ultimate Boot CD available for download from
http://www.ultimatebootcd.com/
You can download this burn it to a CD and then set your BIOS to boot from a CD and run the Diagnostics to find out if you have any problems on the system.
But just as an after thought I would try running CHKDSK first just to make sure that you do not have any bad sectors on your HDD which could be causing this problem.
Col