When you say you "reformat" your computer.
Are you actually running the command "format /U" ?
Or are you referring to the process of installing recovery CD's?
Note that although some recovery CD's do reformat the drive, many do not. They simply install over existing installations.
Since I did not see you report the actual error messages you are receiving, unless I missed it. What is the exact error?
Have you after reinstalling your OS, run scandisk thorough? Since you stated now that your getting an error with different programs. Sometimes, you can have a bad disk sector which will cause an error.
After reinstall, then run scandisk. It can take a very long time to check for bad sectors.
If all else fails, maybe you might consider going with an alternative to Yahoo Messenger. Try GAIM, which is a multi-protocol messenger. You will still be able to use the Yahoo IM service, but without needing the Yahoo software