CPU randomly tied up at 100% use

Windows 2000 professional/amdk6 proccessor/128 mg ram/ 2 20 gig maxtor hard drives.System is slow. Task manager shows explorer at 75 to 95% alot of the time, even when system is idle.It shoots up to these #’s randomly. Can’t figure out what is accessing cpu.Ran norton system works,scan disk & defrag.Removed all unused programs from drive and registry. Task manager shows 33 processes running all the time.Is this normal. Help!!