It is absolutely not a problem with your CPU. Your CPU is fine. If you get a faster CPU that will just result in a faster CPU being used 100%.
It is probably one of:
- some web site you visited is running a Flash script, or a Java program, or a Javascript program, or something of the sort. Try closing all of your browser windows and then, if any Firefox processes are left, kill those. If that fixes it, see if there isn't a newer update for Firefox.
- you're using either Norton or McCaffee Antivirus. Get rid of it and install something else. I'm not kidding.
- plain old Explorer (the desktop / file browser, not IE) can do this. One common cause: It is busily creating thumbnails for all of your pictures and video files. Turn off the "thumbnail cacheing" option in Explorer.
- some program you have installed has something that it runs all the time. This becomes a first suspect if the problem shows up immediately after you boot and log in, as opposed to after a period of extended use.Use task manager (be sure to click 'show processes from all users') to find which process it is. Then either uninstall the app that process belongs to, or else (much more risky) use msconfig to remove it from the startup list (things that run automatically when you boot or log in). It's risky because a lot of people mistakenly disable their machines by removing from the startup list things that do need to be there.
- you have some sort of malware (commonly called a virus, but most malware these days isn't a true virus - doesn't matter, it still is bad). Do a complete scan of your machine with at least three top malware scanners. I rec malwarebytes, Microsoft Security Essentials, and a demo version of eset NOD32.
Hope this helps. If not, I would suggest asking in the Microsoft forum at arstechnica.com . Lots of VERY knowledgeable and helpful people there, and unlike here, we can post screen snapshots and other helpful things.