A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
It would be nice to know why these things happen and not have to clear out and re-install!
Why? Office applications have been flakey since "Day 1", back in the 1980's! They are so flakey that they number one trouble shooting tip from MS Support is "uninstall and re-install". The Office applications have actually been explicitly written to support that "fix technique". When you do a "normal" uninstall from the control panel, the uninstaller leaves behind "settings files" like normal.dotm and custom spelling files and also registry entries. The installer checks for these files and if they are found it leaves them in place so you don't have to redo your the customization to personalize the installation.