Please run 3 different download/upload speed tests for both computers and report back with the results. Choose test servers that are geographically closest to you.
What kind of Internet service do both PCs have ? Cable modem, dial-up, DSL, fibre ?
What kinds of connections does each PC have ? Wired or wireless ?
f both PC's use the same type connection (both wired or both wireless) then it can't be a router issue since your mum's downloading speed if OK.
What manufacturer and model motherboard are you using ?
Have you gone to the motherboard manufacturer's web site and checked for updated BIOS and networking drivers ?
_________________________________________
Not that this is causing the slow downloads, but Norton IS is a real
system resources hog. You will be much better off uninstalling it and running either of the free Avast or AVG anti-viruses.
Avast
AVG