I don't think you can... I know at one point in the past year, they made a change and then forced down the new version when msn messenger opened. I don't remember what the new version was when it happened, I just remember that I was very annoyed by it, and my msn messenger has never been the same since.

Other than that, as long as it is giving you a choice to upgrade or not, you should be able to ignore it... at least for a while.

-tg