Yahoo messenger blinking out at start-up

I've looked everywhere and I still can't find any help regarding this problem. My system is telling me that it's already installed but when I click on the Yahoo msg icon, it blinks on then shuts off. The "off" smiley stays on my taskbar but when I move the cursor over it, it disappears.

I've tried un-installing it then doing a re-install, but that doesn't help.

I'm at my wit's end and unfortunately not very tech savy so if someone can help and keep it simple for an old dinosaur like me to understand I'd appreciate it very much.

Uninstall Yahoo, then delete all Yahoo folders from your computer (under "Program Files" and "Documents and Settings"), remove all Yahoo registry keys if you like, reinstall.

Thanks for pointing me in the right direction.

I think something screwy might be going on. When I went into the "Program Files" and tried to delete the Yahoo file it gave me this message:

cannot delete pubmod.dll: access is denied.
make sure the disk is not full or write-protected
and that the file is not currently in use.

So now I'm back to square one again. What can I do to get rid of these files if it won't give me any other option except the one above?