If your virus/spyware free, there are a couple things you can do get your computer working faster. If you using your computer for normal stuff and not gaming. You don't always need more RAM you just need to perform a few maintenance task.
First, if you haven't cleaned your registry in a while. This is necessary. Your registry holds all the information regarding updates, installs, un-installs etc. Each one of those events produces a key. That key needs your PC's resources and free space. So, if you clean your registry, you make more resources available for speed. Do a registry scan if you got more than 20 errors you should clean it.
2. Clear your Windows cache files, this is your temporary file. It can grow up to a large volume and stop your computer from using RAM properly. In a sense slow it down by preventing it from using free space.Here is how:
1. Quit Internet Explorer, Firefox, or any other web browser
2. Click Start, click "Control Panel", and then double-click "Internet Options".
3. On the General tab, click "Delete Files" under Temporary Internet Files.
4. In the Delete Files dialog box, click to select the "Delete all offline content" check box , and then click OK.
5. Click OK.
3. Manage your startups. Your excess CPU usage is due to too many programs running in the background. You can cut these to barebones by alter your msconfig. Here is how:
1. Open your Start menu.
2. Click Run
3. In the command screen type "msconfig.exe"
4. In the "system configuration utility",click either "service" or "startup" tab
5. Uncheck all programs that your are no longer using.
6. Click "OK"
Other great tip is increasing your virtual memory to simulate more RAM. This will give you a bit more resources for multiple program use or CPU heavy programs.
Hope this link helps. Shows you how to do everything step by step. You should get a least 60-120% more resources and speed
if you follow the tips.