Hi!
Since I Update to windows 10 the fan stay on all the time and around 40% of memory is used all the time, even if I'm not using my surface.
How can I solve this?
System (aka ntoskrnl.exe) is using too much RAM in your case. On my 4 GB RAM/128GB storage SP3, ntoskrnl.exe is only using 100MB of RAM and I consume about 1.7 GB of RAM with 15 Tabs on IE, mail app, and playing iheartradio app in the background.
I wouldn't mind about the high RAM usage. As someone already said unused memory is wasted memory, Windows10 ram management is a little different and efficient if I might add. Have a look at this article. Why Is Windows 10's "System" Process Using So Much RAM?