Secure Network Using XP Server 2003 and wireless router

hi every body,

I want to make a secure setup in my office to access only emails. I wanted to stop downloading, streaming, chatting etc

I have Windows XP server 2003, wireless router and DSL router. Right now this server is used as a access point.

I configure the domain server and dhcp server. Now i want to secure Internet access to users.

Please tell me how i should proceed.

Thank You