CPU maxed out!

My Dell 8200 is only a year old, 1.9gz pen-4/256 mb ddr memory. Everytime I try to open or download a pdf file, perform disk clean up, defrag or use any of microsoft’s plus screen savers the cpu pegs out at 100%. The machine slows to a crawl and thefan gets louder. I have adequate ventilation space and i never had this problem until just a couple months ago. My os is XP home. I have done a couple of restore points in the past. Any ideas? Thanx Angelljr