Is this a Scam?

I have been getting a lot of phone calls for months now saying there is something wrong with my pc. Last one with a Indian accent and real aggressive ( I hung up and he call me back)  said my computer had been hacked and my Norton doesn't protect against this . I wanted to know how he got my phone # . He said he had my PC license # and phone from when I registered my pc. Gave me a web site letsassist.net.  I think it's a scam but have some questions

1.Would Norton know and inform me if I was hacked and if not how would I find out?

2.How would this guy get my numbers?

3. What would be the best way to double check to insure I haven't been hacked ?

4. If I leave my PC on line idle for a period of time I get a popup from Microsoft asking if I want help , does this have anything to do with my phone calls ?

Thanks for any advised or help in advance .   Bart