Yes, The currect version (3.62) works with Windows Messenger 5.1 series and even with the 4.7 series. Maybe there are just a few features that don't work, but at least 90% should be ok.
But Messenger Plus! Live (4.0) that will be released pretty soon does not the old Windows Messenger series, you will need at least Window Live Messenger (WLM) for that and Windows XP. But if you don't have Windows XP (or higher) or you simply don't wan't to upgrade to WLM then you're free to keep on using Plus! 3.62
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