Serious problem with high server load (dos attacks?)

[clinton4]

Hi,

I have a big problem with high server load on a server running Plesk 9.2.1

I suspect that someone is doing dos attacks and I can not locate the IP addresses that carry out these attacks. I'am therefore wonder if there is someone who can tell me, if possible, how I can find out which IP addresses that is causing this? And is there a way to prevent this?