I bought a d-link 624 router, I'm having security issues with it.?

I have my desktop and my wireless laptop in my network. If i use the security thru network magic I can't get on the interet with my laptop. When i turn it off both computers can be up and running. I don't like leaving it unsecured, and I have looked and looked for the answer. By the way I bought this router when my son lived with me for his laptop, I no longer have the manual.