Problem opening Excel Sheet created from MS Access 2007 (VBA)

First: I am not an expert at development in VB, VBA or pretty much anything else. I am busy obtaining a Diploma in ICT - long way to go still. We are using Office XP Professional on most of our PCs at work, as it was supplied via a license agreement with Microsoft (Department of Education). I have been using MS Office 2003 (my own licensed package) for some time now, until recently when our institution (at last, on its own) upgraded some of the computers to Office 2007, of which mine was one. Okay, we went through the compatibility problems and all - at least I thought we had most bases covered... My problem is that I use Access to create a mark sheet for Educators. It is an Excel object created from within Access through VBA and now, some users (running Office XP Professional) cannot open any of their custom Excel sheets. A message appears saying that the format of the file is not correct/supported and then it just opens a load of garbage. I am able to open the spreadsheet just fine in Office 2007, but (prior to opening) it reports that the file is "in a different format than specified by the file extension" (probably an XLSX-file with the XLS extension forced onto it - I think), asks whether I want to open the file and then opens up fine. BUT not on my Office 2003 at home (same as Office XP). Has it got anything to do with COMPATIBILITY, the references in VBA, or is it something completely different? Is there a way that I can tell VBA which format/version to use when creating the Excel sheet? I am willing to post some of the (rather poor) code online, if required.