Steps to reinstall Yahoo! Messenger using Safe Mode with Networking in order to avoid any other programs that may be interfering with the installation.

First, download the Yahoo! Messenger offline installer, but PLEASE DO NOT RUN IT YET. Just leave the executable on your desktop for now. 

Restart your computer using Safe Mode with Networking:        

1. If the computer is running, shut down Windows, and then turn off the power.        
2. Wait 30 seconds, and then turn the computer on.        
3. Start tapping the F8 key. The Windows Advanced Options Menu appears. If you begin tapping the F8 key too soon, some computers display a "keyboard error" message. To resolve this, restart the computer and try again.
4. Please make sure that the Safe mode with Networking option is selected.        
5. Press Enter. The computer then begins to start in Safe mode with Networking.   

Uninstall Yahoo! Messenger completely:        

1. On your keyboard, press <Windows> + <R> to open the "Run" command.        
2. Type in appwiz.cpl and press Enter. This opens "Programs and Features."        
3. Select Yahoo! Messenger and click Uninstall.        
4. Press <Windows> + <R> again to open the Run command.        
5. Type in %LocalAppData% and press Enter.        
6. Delete the "Yahoo" folder.        
7. Press <Windows> + <R> again to open the "Run" command.        
8. Type in %ProgramData% and press Enter.        
9. Delete the "Yahoo!" folder.     

Now, run the Yahoo! Messenger installer you downloaded using the instructions above. Once you've reinstalled Yahoo! Messenger, please restart your computer again.