High CPU Usage

Just brought a dell 1012 mini and only really after a couple of weeks did I notice that it would struggle alot with simple tasks like browsing and writing an email.

So I checked resource monitor and there was something called  "system interrupts" which was at 40% CPU.

So now I found the problem I needed to fix it quickly as I was abroad. So I brought an external cd drive and reinstalled the laptop (Formating all the drives)

It still happens.

It has a certain pattern which is that when I turn on the computer its already there and its at 0.5 CPU usage and the longer you leave the laptop on for the Higher the CPU it uses.

I've updated all my driver and reinstalled the OS, I just can't understand what else it could be?

Any Ideas