A friend of mine was having trouble logging into his Yahoo Messenger account with the official Yahoo client on Windows. She had uninstalled/reinstalled various versions of the client, which didn't help. All she was receiving was a "Wrong Password" error (which she didn't get when logging into her Yahoo account on his browser and even using browser-based version of the official client, so the password isn't actually "wrong" and the messenger account is not inaccessible). Thinking that something is definitely wrong with the client, I told her to use Pidgin. She downloaded it, just to end up with the same result.

This is really weird, and I'm pretty sure it's not a case of PEBKAC. Anyone has any idea what might be causing this?

check caps lock and keyboard mapping
(try typing the password in a text editor to see if its what you think you have typed)

otehr than that, try gaim for windows,

To make sure, use some copy/pasta! Write down the address in something like NotePad, copy it, use it to login via webmessenger. If successful, copy/pasta the password again and try it with Yahoo MSG/Pidgin.

No idea what could be wrong, and I have *zero* clue at debugging Win32 execs.

@ dasy2k1: Pidgin is the new name of Gaim

When I say I'm sure it's not PEBKAC, it means all these methods (along with many more) have already been tried. But Thanks anyway.

I guess you already scanned the computer for viruses and the like, if not, try this, a friend of mine had a problem like this one...
Try inserting a linux live cd, such as knoppix, and log in from there.
Try installing something such as ethereal/wireshark and analyze exactly what package the program sends. I don't really remember if the password is encrypted, but at least you can get a clue as to what is happening.(aka to check what is sent to the server and what response it gets).