Do you have it here?
C:\Windows\System32\XPSViewer\XPSViewer.exe
Do you have anything similar in these Registry locations?
HKEY_CLASSES_ROOT\XPSViewer.Document.1\shell\open\command, Default Value REG_SZ with a Data Value "C:\Windows\System32\XPSViewer\XPSViewer.exe" "%1" %*
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\XPSViewer.Document.1\shell\open\command, same details as above.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\App Paths\XPSViewer.exe, Default Value REG_SZ with a Data Value "C:\Windows\System32\XPSViewer\XPSViewer.exe"
View and Generate XPS
http://www.microsoft.com/whdc/xps/viewxps.mspx
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