SQL Server Setup has encountered the following error: Method not found:'Boolean Microsoft.SqlServer.Configuration.MsiExtension.MsiExtensionMetadata.get_IsSlipstreamOrPatch()

Hello,

When I am installing SQL Server 2008 on win7, I encountered an error: SQL Server Setup has encountered the following error: Method not found:'Boolean Microsoft.SqlServer.Configuration.MsiExtension.MsiExtensionMetadata.get_IsSlipstreamOrPatch()‘

What's the metter? What can I do to deal with it? 

I need your speed answers.

Thanks,

Vicky Song