Probably no need to reinstall the whole thing.

Have your installation CD/DVD available, choose Start - Settings - Control
Panel - Add or Remove Programs - Microsoft Office [version], etc.

Assuming you're using Office 2003, click on MS Office 2003 in the list of
programs, then click Change - Add or Remove Features - Next - enable Choose
Advanced Customization... - Next - Expand Office/Office Shared
Features/Converters and Filters/Text Converters, and set Recover Text
Convert to Run All From My Computer.  Click Update and follow the prompts.

If you're using Office 2002 or earlier, the steps are similar, but not
identical. But, it's largely intuitive.