Nitrous oxide is in general illegal for use in street cars... that being said there is at least one kit i have seen that is legal because it is used to enhance the efficency of the engine... (it runs at very low psi) but in general it is illegal to use... you can have a system in your car but it must be properley mounted and disconnected from the engine while on public roads...
I'm not familiar with the kit Alex refers to so I'll not comment on it. Regarding NOS kits, in GENERAL...YES! They are illegal IF THEY ARE ARMED, CHARGED AND AVAILABLE FOR IMMEDIATE USE! Meaning, you just gotta hit the button. I run a Zex System on my car and have been questioned before by Police Officers. Once I show that the bottle is empty and the feed line disconnected, they leave me alone. HOWEVER, if you get stopped by a "COP" you're gonna have a rough day regardless because he can pretty much do ANYTHING he want to include towing and impounding the car. Procede with due caution!
if you get in trouble with it,thats your fault..not saying its ok but even if you get a blowout and the tire was bad to begin with and cause damage your in trouble..in 20 years i got pulled over once by the man... he said here just wanted to look... pictures of my street ride with 600 hp setup.