It could be one or more of the following..
1) your system is overheating
2) Windows may be running maintenance in the background such as updates or defrag. This is most prevalent the first few days or so of starting windows and has many updates to catch up on.
3) spyware, malware, virus, etc... Get rid of the 30 day free trial AV and purchase a full version or use Free AV such as AVAST FREE.
I use AVAST FREE and malwarebytes antimalware and I have absolutely NO problems with such things.
There may be other programs that run scheduled tasks such as scans or updates, When I gamed I used to use GAMEBOOSTER, its a free program that temporarily shuts down all programs and services that are not necessary to run windows and stops automatic updates as well.
It is configurable to allow some preferred services that you dont want to stop, the best part is you can restart all those services and programs again with one click and without rebooting the computer.
Let us know what you find