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I never see the point in upgrading my MSN..I just trugg along with my current version whilst everyone tries to send me these flashy thing that I don't want...And they fail due to the awesomeness of MSN 7.5
Plus? You mean like this? If that's what you are talking about, then you should get it. If you have the newest version of MSN, you just download Plus, and it adds all of it's new features, like the colored names, formatting (bold, italic, underline) using BBCode such as , , and (like you do in posts here). You might want to look at the site. It explains the features better than I can.
No, you don't delete MSN. That is an add-on for MSN, not MSN itself. You leave MSN installed. It's very easy to set up (just make sure you aren't signed in to MSN when you install it).