Confused on Where to Start...Need Help!

I already have an Exch2K Server running, with XP as my client. Instant
Messager was not initially installed when the Exch Svr was configured. I now
want to install Instant Messager from the Exch2K CD, but Instant Messager not
an option unless I check the install Exch2K option first. Will this right
over my current configuration of
Exch2K and mailboxes, or will it just install the Instant Messager piece?
AND after I get Instant Messenger installed, do I need to install Messenger
5.X?