If you're looking to download a patch and not read a bunch of technical rambling, skip to the end now.
Although I don't use any Yahoo services other than the simple Java-based games, I had to download the new Windows Presentation Foundation (WPF)-powered Messenger client for Windows Vista. Why? It didn't work with Windows Vista x64 and I had to come up with a solution, obviously.
The installer Yahoo forces you to download is a mini Nullsoft Installer-powered package that merely checks your machine to ensure you met prerequisites, presents options, bla bla -- a lot like the irritating Windows Live Installer.
To bypass all the crap, I found and navigated to their back-end initialization file (thanks Wireshark). It contained links to the Messenger product installation file along with the other superfluous junk like Yahoo's Toolbar.
Trying to execute Messenger out of the box post-install will result in a BadImageFormatException. This is simply because Yahoo.Messenger.YmApp.exe is a .NET executable compiled with "Any CPU" as the target platform (you can verify this by using corflags and analyzing the 32BIT flag). As a result, the 64-bit CLR kicks in and tries to load 32-bit binaries into 64-bit space, and... yeah. The Moon collides with the Earth.
Normally, I would have used the /32bit+ switch to set the 32-bit flag myself but the fact the executable was strong-named/signed didn't help. The only way around this was to reassemble the entire executable.
After some key generation, disassembling, and reassembling, I replaced the offending executable with my freshly baked one. I logged in, played around a bit, and closed it. Hurray! Time for a (re)launch party!
Because redistribution of copyrighted material is a sin, I used bsdiff to generate a small patch, scripted the application process, and zipped it up for public consumption. Don't forget to download the full installer, as I mentioned above.