I get the same "too busy" error.
For pre-loaded games, if they're only missing the .exe file, does that mean we can at least peruse the manual, listen to the soundtrack, watch .bik video, etc.?
Don't quote me on this, but if memory serves, pre-loaded content is delivered in a single encrypted file which is later decrypted and un-packed when the game is finally unlocked.
It's been a long time since I've pre-ordered anything on Steam, however, so I may be incorrect.