Hi,
my Xbox 360 is experiencing ring of death. Can anyone advise me what I can do?

1) I understand MS has extended the warranty period to 3 yrs, but my set is a modded set, is it still under warranty?
2) Because I actually bought my set online, I don't know whether the seal is broken? Can anyone advise me where I can check my seal?
3) If I were to bring it to MS, do I need to remove my modded chip?
PLEASE HELP