MacBook Security

I've had a couple of Macs and have never had any security issues until today. Every once in a while my Mac will tell me that someone wants to access my wireless internet and i just deny it. But today, all day long, I have had a message identifying a user who wants to access my Mac Keychain. Those are all my passwords so I denied the request and turned off my Airport (wireless capability). Do I need to get internet security for my Mac? I never have before, but obviously someone is trying to access my passwords. (I'm sending this email from my windows-based PC which has additional security)

Thanks for your help.