You should be able to log right in to see the main files downloaded provided you get the Steam engine running first on the 64bit edition. Problems are seen if you reformat a drive and go to reinstall the games themselves were you may end up not being able to log onto an existing account.
Talk about "foobar Steam" the HL2 Orange Box saw that problem when the Vista drive here was reformatted no longer being able to reinstall that seeing the duplicate cd key error. Steam never replied to the support request on that. That was the second OB since when trying to setup a second account for the first OBox it activated immediately to the existing HL2 original account where Steam reset the password making that account unusable.
Think that was bad? The game Bioshock was installed on the 7 beta but wouldn't later install on Vista. Toss the game dvd on August 1st when the beta expires! Don't you love the activate once and once only with no options when dual booting?
GGrrr... #wtf$$ !!!