Yes, as long as you haven't had sex, you are still a virgin. A virgin is someone who has never had sex. If you use a tampon, but have never had sex, you are still a virgin.
So why do people ask this question? Girls who are virgins usually have a hymen, a very thin piece of skin-like tissue that partly covers the opening of the vagina. The first time a girl has sex, the hymen stretches and may tear and bleed a little. That is why people say the hymen is "broken."
Putting in a tampon can stretch a girl's hymen and even tear it, but this isn't common. Even if does happen, she is still a virgin until she has had sex.