When you open "Network" in Windows Vista you'll find you do see more machines than you did in XP, assuming other machines on your network are running Vista and are in the "Home" profile. That's due in part to the Link Layer Topology Discovery protocols in Windows Vista. This makes it easier for machines to discover each other and determine how they are connected on a network. For example, do you have your own DNS server on a home network? Maybe but probably not. That makes it difficult for your home computers to find each other and discover names. They leverage older protocols that are not as reliable. LLTD is meant to help solve those types of issues.
So what about the XP machines are your network? Can you discover them? You can but you'll need to install the LLTD hotfix available for XP. This isn't something you would want to deploy enterprise wide without first doing a lot of testing as it does obviously change how machines are talking to each other. There is a LLTD specifications guide that should answer most questions. There is also a write-up by the Cable Guy.
Link to Download details: Link Layer Topology Discovery (LLTD) Responder (KB922120)