More than likely something like anti-virus software or possibly malware is running in the back ground and increasing the CPU usage. Add on gadgets have also been known to install improperly and while they work fine they constantly have excessive CPU usage. If you've installed any new programs or utilities lately that would be a prime candidiate for investigation in Task Manager.
Building a reliable PC involves more than just assembling the parts. You need to be able to configure all of the BIOS settings appropriately. This can be quite involved and frustrating as it can require a lot of trial and error with stress testing. It is however often the only means to get a 100% reliable PC.