connecting MSN messenger and Messenger?

Hi all,
sorry if this is a stupid question - but MSN messenger and Messenger
are 2 completely separate programs correct???

Are they compatible? I was trying to connect from Messenger (on XP)
to a ME computer with MSN messenger - and had problems.
The chat worked, and i could briefly hear them, but they
couldn't hear me...then i just cut out...

should I just install MSN messenger as well?
Are the versions backward compatible (i.e. if i get the new
version the other person does not need to uprage as well?)

Would a newer version of MSN messenger work with Messenger...

thanks in advance!
Mark