Are you sure it's absolutely necessary to have a second account for use when not on the network? In my experience, a typical domain account will 'cache' the user credentials such that the account can be used from home etc. when the user is away from the network. Having two accounts used by the same user for the same machine would seem to be highly frustrating.
To your point about the "auto complete" issue. I did a little bit of Googling but came up with nothing definative unfortunatly. I suspect that you or your administrator made a choice early on when using one account or the other that instructed Windows to use or not to use auto complete as the case may be. For the life of me I cannot figure out how to undo these changes after they have been selected and the only thing that I can find for Word or Office specifically has to do with using auto-complete in a document, not for the file selection. so I suspect that is a global XP thing.
here... if anybody has the auto complete change covered it's bound to be Kelly's Korner.