100%?

I have a amd anthlon, it has 512mb of ram, windows XP pro, and Norton using about 280mb of my ram. I have recently been having trouble with the computer, it has had trouble running in windows and in yahoo mail. Once I get on the internet it works slow but downloads pages and pics pretty well but still not as slow as when moving around desktop etc. I have run malwarebytes anti malware thinking I caught something.
now what I found interesting is that when I had the task manager up with nothing really moving around, under cpu usage it was at 100%! Now I confess to not know sh&t about computers, but that isn't right, like now as I move around the internet, the usage varys greatly and rapidly, like you would expect.
Does anyone have any ideas why this is happening intermitently to my computer. Right now it is running well.
I have tried very hard to download and use memtest86+ and finally made a copy on cd,(still zipped) hope that is right, but have not been able to use it to boot, i really don't know how.
Any advice about the 100% thing would be greatly appreciated
Doug

you can click the Processes tab of the task manager and look down the CPU column to find the .exe that is using the CPU up. You can then use the search feature to search your computer to see what the .exe is from. Then depending on what it is you may be able to just delete the .exe and the problem is solved.

I´d remove Norton and install something like Avira or Avast. (both free)
Then scan.

Have a look at CCleaner for tidying up.

Also hit control,alt.delete. at the same time. Here you can see what process is running ,how much memory it´s using as well.

Norton Anti-Virus is known to severely decrease performance in computers. The software is written very poorly. Download Avast! Anti-Virus as Mr. Soft suggested. It is much better than Norton.