As it states on the subject line this is an error that would cause when the system is trying to load assembly due to the permissions, the second part of the subject also needs consideration that server may be running out of resources & security issue...
After I knew about the SQL 2008 RTM release news I have blogged SQL2008RTM-official here and as usual I have attempted to download the setup media from Technet which took a while (3 hours) to download the file, due to the obvious reasons of MSDN/TEchnet...