From what I can see, you're the first person who's experienced confliction with Norton Internet Security and Messenger Plus! Live, so I can't find any information about any past experiences.
If NIS is blocking Plus!, then yes it will disappear once you install NIS again, however you could do some tests for us to see what the problem is exactly...
- Have you had NIS and Plus! running together fine, until now?
- When you install Plus!, does NIS alert you about it? If so, what does it say? Please screenshot.
- If you just disable NIS (rather than remove), does Plus! work?
- Have you tried adding Windows Live Messenger to the allow list?
Someone
here posted in 2009 about similar confliction problems with color codes and they had NIS, however the problem turned out to be something else; so it's strange why removing NIS fixed the problem. Maybe this confliction is new.
/ 
