Open up task manager, go to the processes tab, click "Show processes from all users" and see what the name of the process that is using your CPU is.
Also, i would never pay for an AV program these days when the free ones like Avast and AVG are very good and free for private use, the home version is basically sponsored by the corporate version and provides the same level of protection. I was using AVG for a while and switched to avast a few years ago, neither has failed me yet.
Do you have an AV program running....... this symptom can be a virus..... I have also seen it occur when an antivirus subscription expires. In several case, I have been able to restore normal CPU usage by removing the offending AV program......in my experience, Norton and TrendMicro have been the biggest offenders here.
Have you checked Task manager, clicked on the CPU column header to rank the services by CPU usage ? See what service is using all the CPU cycles and then search the web for an explanation of that the process is.
What I would do is:
1. Search out those services eating those CPU cycles, find out what they are and uninstall the programs associated therewith if you don't need them.
2. Bitdefender has a fully functional full year free trial period and is one of the top AV / Malware suites available..... received AV-Test Top Choice Award in 2011 and 2012
3. Disconnect PC from internet / network connections.
4. Turn off all services associated with ya AV program and reboot. Take particular attention to make sure active scanning is disabled as well as all scheduled scans. Reboot, making sure afterwards that none of those services restarted.. If CPU usage is normal, ya found ya problem...... it was the AV program
5. Install BitDefender ....run a full scan. If someone told you that you can not have two AV programs installed at the same time, you have been misinformed. As someone who managed on line download libraries going back over 20 years ago, I can tell you that before releasing any uploaded file for library download, sysops were required to scan the file with at least 2 AV programs. You can certainly have two AV programs installed. I have never been w/o 2 going back to 1990.
What you can't do is have two AV programs running at the same time.....
Let me however provide an alternate viewpoint to hunter .... the freebie AV's are just fine for single users. I used em all over the years, Spybot, Lavasoft, Avast, AVG, etc...... but got tired of running 3 or 4 programs and having to manually start those that didn't do everything automatically. But the real kicker was that I have 12 PC's in my home and I got tired of going from 1st to the next to the next and do manual updates; yes I will pay $5 a PC per year to be relieved of that task. I first moved away from the freebies with F-Prot a nice little AV program that used very few resources and cost me just $3 per seat for a site license. I now use another suite that combines a firewall, AV and malware scanning, encryption and several other security features, ....it costs me about $6 a seat per year ... and no it's not BitDefender
That long free trial period period for BD comes in handy when troubleshooting someone's PC. Regardless of what PC software they are currently using, I can d/l BD, solve their problem w/o costing them a dime. Of course how they gonna get by after the trial period is up to them.