XPS documents will no doubt start to surface on the web over the course of this summer. If you're not yet ready to load Vista as your day-to-day OS there are options for you to open and read .XPS files in XP.
The XML Paper Specification Essentials Pack (Beta 1) provides a standalone viewer that will register itself as the default for XPS. You can also load the February Community Technology Preview of the WinFX Runtime Components to open .XPS files in IE6 or later.
What about the XPS Annotator? My friend recommended to use the XPS Annotator. He says XPS Annotator has many advantages compared to other XPS Viewer.