A good processor with plenty of ram and a good hardrive.
If its not Spyware or Ad-Ware not really sure what it could be.
You said your CPU usage bounces around between 40 - 100% It could be a program running that is slowing down your computer so much.
Enter 'Task Manager' and click the "Processes" tab and see if there is any one program that is running that is using more then its fare share of the CPU usage. That might be the cause of the slowness.
(Right at the bottom of the page there will be a "System Idle Process". Just ignore that and how much its using though.)
Its thats not the problem, your guess is as good as mine.