Receiving appointments in Outlook from Entourage



I have a network of 3 PC's and two Mac's.

On my one mac I have Entourage Office X installed. On one PC I have Office 2003 installed. On another pc I have Office XP installed.

The issue I'm having is with sending meeting requests from the Calendar.
If I send it from Outlook XP to Entourage I can accept and decline just fine in Entourage
If I send it from Outlook 2003 to Entourage I can accept and decline just fine in Entourage.
Now when I send from Entourage to 2003 or xp, On both outlooks, the appointment shows up as a regular email with text about the appointment and no option to accept/decline, thus it does not get added tot he calendar either.

Is there a setting in Entourage or Outlook I have to change in order to send or receive an appointment correctly? Please help, thanks!
I'm not sure, but definitely let me know if you find out !!!
Personally, I want to know how I can create a meeting request (or .ics file) that I can send to a pool of MAC AND PC users so that they can all add it to either their iCal or Exchange calendars. This should be possible.. but I don't think it is yet.. :-(