Thanks for your help. But Microsoft does not give XNA devs the same access to player data that other games like Mass Effect does. It is not within the XNA framework. I've asked on other forums and it seems the only option is possibly a password system.
As for PC, it may be possible it I could simply specify the directory where the save file is stored. But I'm having trouble finding the correct syntax for that. I can save in IsolatedStorage or in SavedGames but I cannot find a way to specify the sub directory.