I'm doubtful that the issue is related to the memory. Though is it two sticks? The motherboard is designed for dual channel DDR2, meaning memory should be installed in matching pairs for best performance. If you have an odd number of memory modules, try removing one, thus making it even, and see if it helps (again doubtful, though it will likely make the computer run better anyhow).
First I would check what is using that CPU. Open task manager (ctlr+alt+del, choose "start task manager" and go to the Perfomance tab. at the bottom of that pane open the Resource Monitor. Here it lists each of the major components and what they are doing. Expand the CPU list. The program/function taxing your CPU shoul be the first one listed.
If its a program, close the program and see if it helps. If its a file that you don't recognize, google the file name (search file type filename and extension i.e. abcd.exe). If it is a specific file/program go to the processes tab in the task manager, right click on the matching file name and select end process and see if that fixes things.
Hopefully its not a virus. But you could run Norton or another program like it to see if it finds anything.