Why the hell not? Does the user not have the right to choose what he does and does not have on his computer? I could understand it if someone was trying to delete a file that is needed by Windows, but Windows Messenger is in no way crucial to the operations of Windows code or the OS itself, thus it should be up to the user if he finds any need to have Windows Messenger on his computer. If not, then he should be able to uninstall it, without the need for some code, and the same goes for MSN.
Anyways - just delete the main .exe file of whichever you want to remove - and show your teachers - if they have any sense at all, they would realise that neither MSN or Windows Messenger would need that .exe file to run.