I have discovered the problem. I -was- being hacked.
...by Norton Internet Security.
My subscription with them (FINALLY) ran out and they locked access to all kinds of things - presumably as a way to scare me into renewing.
I had to download a specific program from Symantec's bloody WEBSITE in order to delete the programs from my computer. Lo and behold, I restart my computer after running the program, and Yahoo Mail works like a charm.
My hatred for Norton has reached an all-time high. It's too bad it came preinstalled on my computer.
Thanks for your time, everyone.