mtassin,
I encountered exactly the same problem (albeit on a Windows 2003 Ent Ed R2
SP2 installation). After a bit of digging around, I came across [1] which
seemed to imply a repair of the .Net 2.0 framework would resolve the problems.
It did, but I then received the same error for the
Microsoft.BizTalk.Adapter.MQS.dll assembly. After some further digging around I
came across [2] which seemed like a bit of hard-work.
On an off-chance, I connected my machine to the net (I'm running the
environment in a VMWare machine and it was only connected locally) and the
installation worked without error – I am now working through the configuration.
I don't know if connecting to the 'net did anything, but something worked to
resolve the error. I would suggest you look through those two links and see
whether either of them work for you.
Good luck!