Maybe a long shot but could be the CPU is getting overheated and clocks down to prevent that, and thus slowing everything down.

Google around and see which CPU temp software works with your CPU and check the temps both at idle and load. Also see if you can find what the recommended temps are for that CPU.

If you have good cooling for the computer you might wanna open it up and clean away dust if it's been a long time.

You might also wanna check if the reason is a specific service running in the background. You can use the software ProcessExplorer for that.

And make sure Norton 360 isn't running any backup-copy thing in the background. I'm using Norton internet security myself so I'm not familiar with the additions in 360.

EDIT: To check if the CPU is clocking down you might also wanna monitor the CPU clock. Google around for software working with your CPU for this also.