I have introduce many XPS tools before, and today I find another great XPS viewer, java-axp.
java-axp is a 100% java XPS file reader. The goal of the project is to create a java library for parsing, reading and rendering XPS documents in java.

The project currently consists of two core OSGi bundles, xps-core and xps-swingviewer. The xps-core bundle provides a document model and a file input layer. The xps-swingviewer bundle provides several Swing components to render XPS documents.
for details, please visit http://code.google.com/p/java-axp/

Software Imaging have released a new XPS viewer as a free download. The viewer uses Software Imaging’s own PrintMagicXPS technology to render the XPS documents, which differs from other XPS viewers because it is developed in C++ native code, and therefore has no dependency on .Net or managed code.

Features include:

• High quality rasterizer which leverages Software Imaging’s PrintMagicXPS technology
• Full compliance with the XPS 1.0 specification
• Easy to use and view your XPS documents
• Priority given to the rendering of the first page of a multiple page XPS document
• Multi-threaded application for improved performance on multi-core systems
• Developed in C++ native code, therefore no dependency on .Net and managed code
• Two rendering modes: Aliased and Anti-Aliased
• Zoom and rotate views
• Colour management

Supported Operating Systems

• Microsoft Windows Vista
• Microsoft Windows Server 2003
• Microsoft Windows XP SP2

Prerequisites on Download Page

To run XPS Viewer on Windows XP Service Pack 2 or Window Server 2003 the following components will need to be downloaded and installed:

• Microsoft Core XML Services (MSXML) 6.0
• Microsoft Windows Imaging Component (WIC)

btw, Windows 7 Will Include New Win32 API’s For XPS, and the new API’s will be:

• XPS API provides creation, manipulation, reading, writing and other services for XPS documents and print streams
• OPC API provides creation, manipulation, reading, writing and other services for Open Packaging Conventions-based file formats, including XPS, OOXML and an increasing number of industrial strength third party file formats
• XPS Print API provides a new entry point for applications to benefit from the enhanced XPS Print Path, irrespective of whether the final output device supports XPS

reference,
http://digitaldocuments.debenu.com/2008/11/12/windows-7-will-include-new-win32-apis-for-xps/
http://digitaldocuments.debenu.com/2008/10/01/new-xps-viewer-from-software-imaging/
http://softwareimaging.com/products-services/XPSViewer/index.asp

I tried to install XPS Essentials Pack(aka XPSEP) many times on my PC, but all failed, the error message is,

“There is a problem with this Windows Installer package. A program run as part of the setup did not finish as expected….”

My PC has installed Windows XP SP3, and .NET Framework 3.5.
After searching, I got the following solution,

There is a brute force solution to that :-)
1. Make sure you uninstalled the .NET 3.x things (no %windir%\system32\xpsviewer folder anymore)
2. Make sure you have WIC installed or install it.
3. Get yourself Orca, the MSI database editor from MS (you may have to get a Windows Platform SDK in order to get access to the Orca.msi); I used Orca 3.1.4000.2049
4. Make a copy of your XPSEP MSI.
5. Right-click on the copy – “Edit with Orca”
6. Go to InstallExecuteSequence.
7. Find the Row labelled WICUpdateInstallAction – Right-click – “Drop Row”.
8. Save.
9. Double-click the MSI copy you just edited.
10. Hopefully, the installer should run through fine.
11. Reinstall .NET 3.x and all other things you uninstalled beforehand.

unlucky, I can not successfully uninstall .NET 3.0, So I tried on my Virtual PC, it did not install SP3 and .NET Framework, and succeeded.