Having problems installing ExpressViewer on WinXP

I'm having problems installing Autodesk's EpressViewer on my WinXP stations. I'm getting the message:
There was a problem registering this file The installation will be cancelled
But the Installation does continue after I click OK on the error message (above)
Then then I go to open ExpressViewer I get the messages:
Invalid Class String
Can any help with this problem?
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