That both programs give this kind of error isn't related at all. That is, is is not because Plus! can't be uninstalled that Messenger can't, or vice versa.
What can relate these two errors together is an error on your harddisk though. Which makes that both (and maybe even other) programs became corrupt.
Anyways, reinstall Messenger Plus! and after that try again to uninstall it:
1) Reboot your computer
2) Clear your browser cache. Instructions can be found here:
https://kb.iu.edu/data/ahic.html
3) Disable your virus scanner and/or other protection software which might prevent programs from installing.
4) Redownload Messenger Plus! Live (from
here) to make sure you have a proper non-corrupted installer
5) Run the installer and install Messenger Plus! Live.
6) After that, uninstall it again by going to the 'add/Remove programs' panel
7) Reboot your computer
Do the same for Windows Live Messenger: clear cache, redownload, reinstall...
PS: and check your entire hard disk for errors... you never know...
/ 
