CPU usage is obvioulsy a direct factor of what is running on your PC. People simply don't realize how many little "*.exe" files are installed and launch each time you boot up your PC.
Here are some tips for improving speed and performance.
A major cause of slow PC's is fragmentation of the hard drive.
You need to defragment the HD at least once a week if not more depending on how much you use your computer, and what you do with it.
One way is to use the defragment utility built into Windows XP. It is slow but does the job rather well. Go to:
Start > Programs > Accessories > System Tools and then click on System Defrag to run the tool. It will take several hours so let it run overnight or simply walk away from your PC for a few hours.
A great program for defragging automatically is Diskeeper. It costs $30 but is well worth it. Go to www.download.com and search for Diskeeper. Read the Publishers Description and the tech and user reviews. It can be scheduled to run automatically as the program deems necessary, and if you are using your PC and the program launches itself, it will take a backseat to whatever you are doing and give you the priority for CPU usage. I think you'll agree it's a good program to have. It precludes your needing to remember to defrag at regular intervals.
Also, another major cause of a slow PC is programs that are running in the background. Here's another really good Microsoft tool. Go to Start > Run > type in msconfig in the box and click OK.
This will bring up a window with several tabs at the top. Click on the “Startup” tab. It will list the programs that are running on your PC and eating up resources. Look them over carefully and turn off any that don’t absolutely have to be running. Simply click on the “check mark” in the little box beside the program name. Clicking on it will remove the check mark. Once you have edited the Startup list, click on Apply and then on OK. Once you have exited the window, reboot your PC for the settings to take effect.
Another good thing to have is a little program called FreeRAM Pro. It is free and is available from www.download.com This program puts a little RAM meter on the System Tray and shows how much RAM you have remaining that you are not using at any given time. I think you may be surprised at how little RAM you have to run other programs. Also, once it is installed you can right click on the little meter, then click on RESTORE and it will open the RAM display window. You can tweak the settings so it will display your CPU usage also. Then just click “Minimize” and then “To System Tray” to return to the Task Bar.
What happens when you run out of or run real low on RAM is that the OS recognizes the low or no RAM situation and starts using a section of your hard drive as memory. This is called virtual memory, or a swap file. The PC is constantly writing and reading from your HD. Comparitively, it is a very slow process. Check your motherboard manual and determine how much RAM each slot can hold. It will also tell you what type of RAM you need. You can also go to http://www.crucial.com and run their analysis utility to determine what kind and how much RAM is installed in your PC. Depend on your MoBo manual to determine the RAM limits of each slot though.
Hope this helps