>>
>>It sounds as if you added the reference via the Browse tab - in general it's easier (and probably safer) to select the Net namespace from the .Net tab.......
>
>Possibly, but I think that was because they weren't available in the .Net tab

Seems odd - IIRC those assemblies would normally be in the GAC and, by extension. listed under that tab.
You could try adding them there yourself to make things easier in the future :
http://www.thescarms.com/dotnet/gacutil.aspx