1. Basic system info is commonly captured at the far end. This includes your browser type and version, OS type and version, and (less frequently) processor and RAM. With an installed client, they can get far more information than they can with simple browser access.

2. On the privacy rights issue: I don't know about Stars, but I've pored over the TOS for Party. When you agree to the Terms of Service, you EXPLICITLY allow them to scan your machine, scan what's running, and even take desktop snapshots. There are no limitations listed on what they're allowed to do with the information, where they store it, etc. While it may work against your privacy, it doesn't work against your RIGHT to privacy, since you explicitly gave up that right when you agreed to the terms.

Mind you, I (like most) didn't realize that at the time I agreed to the TOS. I went back and read it much later!