HOUSTON -- From her family's Montgomery County home, a 15-year-old girl surfed the Internet for horses she hoped to afford someday. And like any teenager, she caught up on gossip with instant messages to her girlfriends.

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Hacker Takes Over Teen's ComputerExpert Offers Tips To Protect Computer From Cyberintruders

But imagine how shocked you would be if a message you were sending to a friend suddenly turned disgusting. Your friend thinks it is you typing those words. That's how this teenager first realized a hacker had taken over.

"I was talking to one of my friends and I looked up to what I was typing, and I wasn't typing. Someone else was, and it was perverted stuff," the hacker victim said.

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The hacker made sure the girl knew he was in charge with annoying tricks like opening her computer's disc drive.

The noise and the movement were a startling invasion of the teenager's private computer room.

"He'd write words in the middle of my screen, like 'I'm in your computer.' And he'd make my screen go completely black," the teenager said.

Then, a chilling reality sunk in. The stranger was also controlling the web camera. It was normally used to send live pictures to someone the teen was messaging online, but he was using it for something else.

"He was like, 'I like your shirt.' Or something like that," she said. "It felt like he was standing right next to me, watching my every move."

The reason the hacker could tell what the girl was wearing was because the family never knew when he had turned their webcam on to look inside their home. The hacker got enough family information from inside their computer that the mother said she's terrified he will show up in their driveway someday.

"I felt he was in my room with me. He was in our room, in our home. He knew a lot about us. Who knows how long he's had that computer on? He turns it on quite a bit," the mother said.

She said the hacker even listened to conversations, using the microphone attached to the webcam.

The words spoken in the home would suddenly be typed out on the computer screen. He also blocked the mother from online gold trading -- the whole reason she bought the computer.

"He knew everything. If I was trading, he could take over my trades," the mother said.

But despite all the fancy tricks, this is not a high-tech hacker you would expect.

"He can be anyone. He can be a teenager that just got into this thing," said Daniel, a computer expert.

Daniel fights cyberintruders for a living. He asked that News2Houston not identify him or his major Houston web security firm so that hackers will not retaliate against him or his clients.

Daniel said it is surprisingly common. The worms and Trojan horse programs that allow someone to manipulate your computer are available to anyone on the Internet.

They are usually disguised as popular movies, videos or songs that people would want on the Internet.

"Those worms are hidden on those things. When we download it, boom. It comes into our computer, infects it all. That's how we make it easy for them," Daniel said.

Now, the U.S. Secret Service is on the case. News2Houston's cameras were there as agents cracked into the teenager's computer. Houston agents were just briefed about these computer takeovers happening around the country.

Now they are trying to trace the hacker's identity since he's trying to lure the girl into an illegal meeting.

"He was like, 'You're pretty hot. I want to meet you. Do you think we could ever have anything sexual?' I was like, 'No. I would never meet you and nothing would ever happen,'" the teenager said.

Experts said inexperienced hackers are not usually looking for teenage sex. They just want to prove they can control someone else's computer.

But there are ways to protect your machine.

"Don't download something that you're not supposed to. Don't even open the e-mail that you don't know who it comes from," Daniel said.

Experts said you should also download a good firewall, which blocks any intrusion attempt.

Follow these tips to protect your computer.

Do not open strange e-mails.
Download a good firewall.
Learn how to use the Internet.
Unplug the webcam when not in use.

This Montgomery County family learned the hard way. They are hoping the hacker's program was erased by the Secret Service, but even so, their computer will never quite feel the same in their home.

"It makes me not even want to get on sometimes because I'm afraid I'm going to be watched or he's going to know all of my private stuff. It makes me not want to do what I always do," the teenager said.