Unsolicited calls claiming to be from DeLL (or Microsoft), to advise you about alleged viral activity on your computer, is a SCAM.   I'm surprised they didn't try to argue with your sister when she said the machine had been checked out. It's unclear how they obtain personal information. Just ignore such calls.
Someone claiming to be dell support has been calling my mothers cell phone and our home phone. They even knew my sisters name, (my sister is the one who first set up the laptop by the way) they said that my pc had suspicious activity that resembled a malicious virus and he asked if I was aware of it. My sister told him " I had my tech guy check it out and that he said it was fine" and he said "okay" and hung upi then contacted someone on dell support live chat on dell website and they said it was a new service offered by dell? is this true. And my pc is perfectly fine, and if this is a new service then is there way to turn it off? its sorta creepy.
"Dell Support" keeps calling me