I use the default Microsoft tools in Vista and Win 7 (Windows Explorer and Picture Viewer, Windows Photo Gallery and Windows Live Photo Gallery) and the star ratings and descriptive tags are all used the same way by all the methods, and they all save the rating into the metadata in the picture file so that other apps can re-use the info you've entered. With Photo Gallery and Live Photo Gallery there may be a slight delay before they pick up changes made by one of the other apps as they have to index all of the files before they spot what's changed, but leave them open for 10 minutes and everything should be in sync. The only exception to this is if your files are set to Read Only, in these cases the info won't be written to the file, and won't be readable by the other tools.
Other third party tools like Picasa keep their ratings info, etc in their own database, and don't (by default) write the info to a file, so if you rate a picture in Picasa, it will still show as unrated in all other tools (eg Windows Photo Gallery).
I use these methods and only use tools (like the Microsoft ones) that write to the file's metadata, as I keep my photos on a share on my home network that can be accessed by our desktop, or any of our laptops and this way all of the ratings, tags, etc that we add can be seen by any of us on any of the PCs in the house, they can even be saved to a disc or USB and given to someone else, and they can see all of the ratings and tags too, if they're interested.