Inspiron 15 3521 slow download speed + 100% disk usage

Hi,

Perhaps I should first say I have gone completely mental because although this laptop is about 4 years old...I HAVE LITERALLY BARELY USED IT. I MEAN LIKE BARELY EVER. LIKE 3X A YEAR.

Not sure if 2 issues are related but I'll mention both:

1. I pay for 400/10 internet through my service provider. (confirmed another laptop gets 400+). My Inspiron gets about 90/10.  I am hardwired to modem.

2. Very often my disk shows 100% usage even when nothing is running!

I've read many other threads on similar topic. I have no smartbyte installed; I have checked everywhere - admin menu, services, program list, task manager. Eve.ry.where.

I constantly run Norton which never finds anything. Today I ran Malwarebytes which found a few items, resolved them, but no performance improvement.

I can't take it anymore.

Any ideas on what may be going on?