I turned on my laptop 2 days ago and it was RUNNING REALLY slowly and I saw that the CPU usage is way up above 80% all the time.
I have had a look and it is the 'System Idle Process' that is doing it.
Why did this suddenly HAPPEN and how can I fix it?
The laptop is soooo slow it's driving me nuts.
ThanksIf your system idle process is at 80%, that is good, not bad. It means your cpu is spending 80% of the time "idle", which means "not doing anything". You should look for another CAUSE.
System Idle Process shows how much of the processor's processing power is idle. From what you have said, it seems that 80% of the processor is idle, and 20% is being USED by other process. Perhaps an AV scan is in progress?

EDIT: Beat me to it, Dias. This could also be CAUSED by hardware interrupts.  If the CPU usage is actually shown as 80+, and system idle process is 80+, there is some problem that is hidden within that process, not shown by the default task manager.
Download a free tool called Process Explorer and have a look, give us a screenshot.