I was having a problem where Process Explorer showed Hardware
Interrupts using up an average of 40-50% of my CPU (and the computer
was dragging).

I think a bad Zone Alarm install may have been the problem so I
removed it. Interrupts immediately dropped but didn't go away
totally, they still pop up around 5-10%, peaking at 50% when programs
load. While this is an improvement, I'm not sure if I am supposed to
see hardware interrupts at all during normal usage. What is
considered a normal level of CPU usage for them?