The only way that I can think of would be to un-install Live Messenger before trying to install Messenger 7.5. Having said that I don't use either. But it is worth trying if that's the way you want to go.
Use windows live messenger and the plus attachment (free, just google for it) and then you can if you go into the options of plus allow more than one instance of it to run. However it is both confusing and also crashes more. I have in 7 days had already 1 crash and one long slow down due to it. Alternatively run messenger live and windows messenger which is loaded by default with XP these do not have problems running side by side.
Hi, to the best of my limited knowledge - Windows Live Messenger *is* MSN updated. One might want to run both, perhaps in order to connect to two different people in different locations at the same time, one of whom might not have WLM or vice-versa. If that makes sense!