I purchased a Dell Inspiron 1525 some time ago with Windows Vista preinstalled for my wife. However after a year the machine was constantly maxing out its CPU. I tried everything, from installing all the Microsoft performance patches running virus scans and spy ware scans with every program I knew. I even searched Dell forums and installed the latest hardware drivers to make sure. I eventually installed Unbuntu Linux on the machine in dual boot mode to make sure it was not the machine causing the problem, and it wasn't the machines fault at all. Unbuntu barely used 5% cpu occasionally it would jump to 100% when using Firefox but other than that the machine was very usable.
So the problem must have been Vista, so I continued to monitor processes especially the svchost process and check what services this "catch all" windows service was running. Eventually I detected one that kept on coming up all the time was Windows Defender (MsMpEng.exe). I decided to disable Windows Defender by going into the control panel opening up Windows Defender selecting options and disabling it. Like magic the machine was down to 5% CPU usage! Opening FireFox, IE and Outlook only used short bursts of high CPU before bringing the CPU down to 10% to 34% usage. Which was far better than the 100% constant CPU usage.
One thing I did note though was that AVG anti virus and Windows Defender seem to be in a constant struggle with each other. Its as if Windows Defender checks a file then AVG goes who that file was touched let me just go and check it quickly and then Defender does the same which may have been causing a vicious circle of CPU usage. All I know now is that its better to be have some common sense when surfing the Internet and installing applications on your machine. Anti Virus software if you keep it up to date should be all you need to keep your machine safe. I know some people may require more protection for their machines but there is probably a better product than Windows Defender out there that doesn't stop you using your machine.
Another program I have found to have been of great use to me is Process Tamer. This little app sits in the system tray and stops any one process on your machine from using more than 100% cpu usage.
I hope this little article is of use to people!