I get the same error trying to install the silverlight aplha refresh or the release candidate. I can uinstall the original alpha release and install it. The more information just gives you the system recommendations page which lists the different operating systems and browsers acceptable which is most. I have windows XP SP2 which the list says is OK, but the refresh won't install. It gets 80% done and says your operating system/browser is not acceptable for update. Or something like that.
The alpha release worked with Orcas beta 1. I'm trying to get silverlight working with ORACAS beta 2.