Router behind Router security

I want to set up a wireless router completely open to the public for use in my business, but I want to secure my business PC, which also uses the same ISP connection, from intruders. I have a wired router but I’m not sure how to do wired router behind wireless router security. I took some suggestions and turned off DHCP which allows the business computer to get online, but I can still see my business computer from my laptop using wireless…which means so can everyone else. Any suggestions? The routers are Linksys (WRT300N) and (BEFS41)wireless and wired routers respectively.

Russell