If all you want is mail, you can do this. Calendars are CalDAV (http://caldav.calconnect.org/implementations/clients.html) and contacts are CardDAV (http://carddav.calconnect.org/implementations/clients.html) Your options on Windows systems is limited.
It is far easier to make Macs work with Exchange than it is to get Windows to work with Apple's groupware solution.
If you only have 5 systems, why not move to a hosted mail solution? Get rid of the headache of managing mail internally and break away from all the single points of failure that you have. Rackspace, Microsoft, and many others provide very good hosted solutions and the headache of mail is no longer yours.