Download and run the Windows Live Messenger installer again. Once reinstalled, it should appear on Add/Remove Programs list. Remove it.
If that doesn't work:
Make sure WLM isn't running. (If it is, exit it)
You need to manually remove it.
To do this, go to C:\Program Files\ and delete the folder called MSN Messenger.
Then download MSN Messenger 7.5 and install it.
Then on Saturday you can install WLM again with Messenger Plus! Live