Since Norton 360 is a full suite of anti-malware tools, it's your choice if you want to disable Windows Defender. You may see a slight increase in overall performance by not having two real-time security programs running at the same time.
As to why you're getting error messages when selecting the links to go to Window Defender's Website and to Microsoft's Malware Protection Center, it's possible that the links are dead because of having Norton 360 (total speculation on my part and nothing definite to back that statement up.) Are you able to access the websites by going to them directly rather than using the links? Personally, I wouldn't worry about Windows Defender since your computer does have alternate protection.