Tech support scam advice

hi all,any help/advice you can give would be appreciated.

my sisters computer was remotely accessed by one of them phone scams where they get you to go to a site and run a script where they gain access.he was in there for ten mins aprox.it was only when he started looking for a credit card that she became suspicious and pulled the plug.

 

should i reset back to factory settings just to be 100% sure or am i being paranoid?

again any help greatly appreciated