Zip is self explanatory.
XPS, however, stands for 'XML Paper Specification' Microsoft's PDF competitor, which, due to Microsoft's inability to make things open, only has an official viewer application for Windows.
Searching around a bit, however, I found something called 'NiXPS', which seems to be the only Mac XPS viewer, and a license costs 75, and apparently the trial version puts the words 'TRIAL VERSION' all over the document in red.
So basically, you're out of luck without a copy of Windows, and you have the folks at Redmond to thank for that.