I have an Acer E1 that came with Window 8, and I upgraded to W10.  It's running like a sloth on downers for no obvious reason that I can see (it's sporting a 4th gen i5 and 8Gbs RAM, so specs are decent enough), but it just seems that everything is a slog - opening Firefox for example, took about 3-5 mins yesterday. I noticed that the HDD is now always almost at 100%, which led me to think of a malware issue, but I ran scans to no avail.  If I do a fresh Windows install am I right in thinking I can download W10 from Microsoft and install, and it'll pick up my baked in Windows key that came with the machine? Or, as it came with W8, do I need to download W8 and then upgrade to W10? Actually, is that even possible any more - the upgrade to W10 isn't free anymore, is it?

Any advice would be gratefully received.  :)