Is something running in the background that is a resource hog? One way to find out is to start Task Manager and click on the Processes tab. For more detail download and run Process Explorer
from the Microsoft site. Another option is to run System Configuration (type "System Configuration" in the Search box visible when you click the Start button and select the program). The Startup tab allows you to see what is being run automatically as Windows starts up. That tool also allows you to selectively disable stuff on the next start to allow troubleshooting.
Hope that helps. If not things get more difficult. With your system as described it doesn't seem as though you have much to play with in terms of fiddling with the hardware, default BIOS settings should work fine and I'm presuming that on a newly built system you haven't loaded some sort of malware before the anti-virus/security suite software fully protected you. If all else fails then start uninstalling software. A good place to begin might be some of the utilities (not drivers) supplied with your motherboard. If you are lucky you'll find the problem. If not then you can always reinstall the latest versions.