I have just had a crash course with Vista and 7. I was sharing files between the two and was presented with the Username and Password question. I don't have passwords setup, so why is it asking for one. Once I figured this out I could also access the PC's with a XP machine.
There is a setting to enable password protection on the PC. By default this option is enabled. With this option enabled only people with a Username and password are allowed access to the PC. This goes without saying even those without passwords are also blocked from accessing the PC.
Control Panel >> All Control Panel Items >> Network and Sharing Center >> Advanced sharing settings