Which NI-488 driver download did you actually download? Was the code using the native .NET interface, or the .NET wrapper? You say that you got version 8.1.20.168 from the eval version of Measurement Studio, and that VS 2005 says you need 8.1.20.168. That seems to imply that you have what VS says you need, no?
In general, the NationalInstruments.Common .NET library is available with either the NI-488 driver download, or the NI-VISA download. You do need to explicitly select its installation, though I suspect you probably already knew that.