Everything done in a sandboxed program is contained in the sandbox, and the sandbox is isolated from the rest of the system. So if the web browser is run sandboxed, and the installer downloaded by it and run, it would be sandboxed too.
Then everything it would install would also be contained in the sandbox, including all of the crap that it offers. And after the install is done and the offer is credited, all of the contents of the sandbox can just be deleted.
It's very unlikely but possible that malware installed in the sandbox may be able to exploit a vulnerability to break out of the sandbox, so it's still important to have antivirus and other anti-malware programs on your computer just in case. But chances are, it will stay trapped in the sandbox, and not be able to affect the rest of the system.
This page on the Sandboxie site has links to helpful information about and on how to use the program:
http://www.sandboxie.com/index.php?HelpTopics