NIS maxes out CPU a lot - why?

Hi,

How can I stop NIS maxing out the CPU on my PC? It frequently shows 100% Norton or thereabouts and it's becoming unusable. And that's when I'm not doing anything bar perhaps browsing or reading email.

Weekly Full Scans take ages too though they do complete with nothing found.

It never used to be this bad. PCs are XP. On my Windows 8.1 ones things are OK.

Any clues or quick solutions? Time to abandon ship to something else like AVG?

Thanks,

Rowantree