Fake Hackers call for pc security

Hey one and all,I am a student of programming. Few days back a friend fell victim to the following scam: A hacker called him up and told " hello, good morning,I'm from Microsoft, your pc has a virus, go to your computer, do X, Y and Z so I can run this program that will clean your computer". My friend believed that guy and followed all these instructions, turned off avira as instructed, and paid 100 dollars by credit card for the "anti-virus clean up" that was " runned into his PC. I am 100% serious. People do fall for these scam , Its really spreading alot. The scammer/hacker was persistent and phoned back to back to check that the victim was doing it everything properly. He was a good talker naturally, as he knew how to speak and what to speakc Just to make my friend believe him. He's cancelled the credit card. He never expects to see the 100 dollars back. So the main question I want to ask is that what should I tell him to do to clean up his computer with a Windows OS?

Its really sad to hear what happened with your friend, actually I work at a well known internet security company. I assume that your friend’s whole PC is hacked by that guy or if not, here ate the measured to be considered for security purposes:
1. He must Backup all the files that you want to keep on a USB drive.
2. He must Backup the whole OS onto a external hard drive, possibly with Macrium or Paragon.
3. Ask him to run Darik's Nuke N'Boot to try to take the PC back to its factory-floor state.
4. He should Install a new OS. Any one that he wants to use, I would dual boot Windows and Linux
5. ask him to get someone to run files in through an anti-malware program before he wants to put it on the computer. if this is a fraud case authorities would ask for the whole OS backup. I don't expect these scammers to ever get caught.
6. I would speak to my ISP, get all the passwords and the IP address changed.
7. if he installs a Windows OS for eg. windows 2000,windows 2003, ask him to install bothunter and run it. even run a full antivirus scan and the Windows security suite scan.

Hello, I think he should call the security experts. Maybe the companies or the official government employees who could help at a lower cost. I really don't be envious of security experts when they are not ready to listen to the client. If his computer knowledge is way very less to do the steps provided by the previous user then I suppose that he should get a computer professional to do some of it for him.
If anyone who can tell any other suggestions then maybe he could get the best possible help.

I think he should not try to do the things that would harm the machine, If the authorities are interested in helping you then anything you do to the machine will might not work. I disbelief authorities would put much effort in this thing. If it is not done properly then the authorized people will have a much more difficult time resolving it. If they are interested to pursue it then I would recommend purchasing a new hard drive and starting it over. I recommend him to Save that hard drive for the authorities. Don’t turn the pc on again with that drive connected.

Hello I Just wanted to say this scam is becoming quite popular, and have received few calls myself from an person in the past week or so attempting this, but the thing that made me laugh is that I don’t even use windows . The only thing you can really do to ensure your friends machine is clean of any threats is to do a complete reinstallation of the operating system. I even urge you that all passwords & accounts stored on the machine are changed from a different computer immediately. The payload is most probably going to be some kind of rootkit.