Could you tell us more about your computer and OS? I am willing to bet this is Windows XP. Altho XP can support 4GB, your system will only show about 3GB due to other operational needs and restraints. See this article.

Next time your CPU is at 100%, open Task Manager, look under Processes, and post what process is using most of your capacity.

My first article of Internet computer safety could be helpful. My second one deals with infected computers; never hurts to run a scan for peace of mind.

Look at this link for suggestions to improve your computer performance. A very good program to clean out junk and temp files is CCleaner. Auslogics has a great defragger.

In CCleaner, under Tools option, you can see what programs automatically start up with your computer. You can disable those that are not necessary to your every day life. Only do this if you know what you are really doing. Do not "delete" anything.