Your XP machines were likely using simple file sharing. XP has simple sharing enabled by default, Vista does not. The file sharing model in effect impacts the ease of sharing and how remote accesses get mapped to local accounts. There are two sharing models - classic and simple. The Microsoft article How to configure file sharing in Windows XP discusses the simple model in more detail. The choice of which model to use is yours based on the granularity of access to shared resources you need to provide. The typical result of two computers being mismatched in their sharing models is much head scratching. Microsoft's propensity to "tweak" default behaviors between operating system releases and various upgrade behaviors when operating in a mixed OS environment give even seasoned network admins their "Doh!" moments.