how can the website work fine in VS but not in deploy???

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Hi!
I've got a silverlight app that runs great from VS 2008 pro, but when
i deploy it, using vista sp1 iis7, i get the javascript alert:

<<"Sys.Preveiw.UI" is null or not an object>>

I feel like since i developed this in VS 2008 beta 2, that the
Microsoft.Web.Preview.dll is the wrong version, but i can't find a
new one anywhere
Any thoughts?
Thanks to all!
.