I've just solved it.

Open an Explorer, go to "C:\Documents & Settings\your login account\Program Data\Microsoft\MSN Messenger"
Here you can find one or more folders with numbers. Inside that folders, delete the file "ListCache.dat"

Now try to login again with messenger 7.5. Cross your fingers!

If it does not work, delete "MSN Messenger" folder. But you will loose all your emoticons, backgrounds, etc, etc. Messenger will recreate the folder again on startup.

P.D.: the folder "Program Data" may have a different name. I've just translated directly from a spanish locale.

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