I am trying to uninstall Yahoo! Messenger from Add/Remove Programs. However, when I attempt to do so, I get the following error message: Could not open INSTALL.LOG file

That is most likely because someone already deleted the folder that contained the program files. In this case you have two options:
- reinstall it and remove it properly through Add/Remove Programs
- or use a good Registry cleaner to unload the unused registry keys (something like CCleaner (http://www.ccleaner.com/)).