Recently there was a vulnerability discovered, because of this a new version of 7.0 has been released with just this minor update. You will be able to upgrade to this with ease (it is advised that people with XP SP2 or above upgrade to 8.1 but 7.0 is also available for those people).
For more information have a look at
Good News for those who wish to keep using MSN Messenger, I do realise this is talking about XP and not 2000 but im quite sure the link at the bottom of the first post is still the same).