On rereading this thread this morning, I don't think the problem is a conflict between Visio and the Visio Viewer -- those two can coexist on the same system. I think the issue is a conflict between Office versions, given that you appear to have both Office 2007 and a beta of Office 2010. (Under some circumstances you can even have two versions of Office on the same system but the beta status of Office 2010 could be a problem here, given that the beta period has expired).
I would get rid of the Office 2010 beta, using
this article if necessary, then try installing Visio again.