Hi DoctorT,
The first thing to try is a repair of Word, via Help > Detect & Repair. It is unlikely that the running CCleaner or a disk defragmenter will have any effect. If the repair doesn't work, see:
http://support.microsoft.com/kb/921541
and:
http://word.mvps.org/FAQs/AppErrors/...artingWord.htm
for various series of troubleshooting steps that will help you track down (and hopefully solve) the problem.