Fast computer becomes slow, unresponsive?

Several days ago my new computer with Win8 started behaving slowly and sometimes freezes. While using the internet (I have Firefox) and another program it sometimes becomes unresponsive and I can not move my mouse or press any buttons. Then I have to manually pull the plug and take out the battery and then restart the computer again and it becomes normal. I never had this problem before though. That is the only problem. I scanned the entire system with McAfee and Stinger McAfee product but both scans turned out clean. I also installed updates. I wonder maybe McAfee is missing something that another program might detect.

Also several months ago two viruses were found on my computer, both of which were taken care of by McAfee. Maybe they were only partially destroyed?

via Bleeping Computer Last 20 Posts http://www.bleepingcomputer.com/forums/t/510808/fast-computer-becomes-slow-unresponsive/