Yes if not done right.
Modding your 360 does not affect anything in a negative manner. Your original discs will work the same as they did before. -Personal experience from modding XBOX 360 for 3 years.
NOOBS
Yes especially online modding.
I wouldn't want to know because you can get a suspension from modding on Xbox LIVE
Modding is illegal on the Xbox 360 and therefore you will get banned.
Modding affects the console.
You can do it by modding your xbox 360. But it is very dangerous as virus can be transmitted through it.
It gives you a console suspension from the Xbox LIVE Service. Where you Xbox 360 console will be usless where it cant be used anymore.
Modding is illegal.
Modding is illegal and you will get banned from Xbox permanently if caught.
There are many sites which provide you with information about modding a xbox 360, however, this is very illegal and should not be attempted.
Currently there is no way short of modding the xbox to put up a web-browser on it.