Well, I fixed it myself.

Running this fixes it for anyone else who may have the same problem:

http://support.microsoft.com/mats/Program_Install_and_Uninstall

Open notepad, create a file with this:

yam.exe --config <your-config-file>.cfg
(your config file would be yam-mmc.cfg if you didn't rename it)

Save as a .bat file (Drop down Save as type All Files). You can name it whatever you'd like as long as it ends in .bat.

Make sure this bat file is in the same directory as yam.exe. Double click the bat and (hopefully) success.

That's what I do anyways, and it's always worked for me.

...or for that matter, won't uninstall either.

I'm attempting to upgrade an old and extremely buggy version of the windows MMC wallet. However, during install I always receive this error message, no matter what I've tried:

http://imgur.com/B8ZT3Ib

I've tried uninstalling the old version first and receive that same error.

Is there a known fix for this?

Thanks.