However I'm running under XP SP2 with latest windows updates, I scanned my whole PC with Escan Antivirus and Ad-aware up to date, and reinstalled my system 2 times... To be more specific, some days after my fresh install of xp and others sofwares, each time I open a page with IE, the process manager shows it takes, with another process called System, 100% of the cpu. And this begins during a period when I didnt install anything else, and all previous softwares I installed still dont take any cpu resources. So the only solution I have for now when this happens is to restore a ghost image of my xp partition, to avoid another time-consuming full install of my system... but I'd really like to know the real cause and solution to this pblm, which I couldnt find yet on all others posts concerning it in this forum.

Is anything plugged into your system when it's running like a portable usb hard drive, pda or anything else? If no try the next step. Try resetting your cmos and restoring the Bios back to it's orginal settings. If you are saying it's going at 100% all the time even when you first install windows a settings in your bios might have got misconfig some how. Iron Sharpens Iron.