XPS(Microsoft XML Paper Specification) is a Microsoft alternative to PDF as you mention .. unfortunately it is far from cross-platform at present. There are no readers for XPS on Linux or OSX as far as I can see.
Microsoft stopped developing and supporting IE on Macintosh ages ago, in fact they never developed an OSX version. The last remains of IE will only run on OS9 and will still not support XPS.
Microsoft are promising a File Converter for their new Office 2007 file formats DOCX, XLSX, PPTX to run on OSX so that useres of Office 2004 can open 2007 documents but no reference on the XLS file format.
For Microsoft's so-called PDF-killer, Open Source alternative there is a complete lack of reader and edit tools for Windows XP and nothing for OSX or Linux either. There may well be readers for Firefox/Safari plug-in readers in due course.
In the meantime you're going to have to print to PDF rather than XPS to exchange files with OSX and Windows XP users.