What version of Spyware Doctor, how many of its modules/options are on/active, and what other security applications run?
Reports I've read about this (no direct experience) suggest overlapping protections can cause this, eg: running (or trying to run) two firewalls, or another AS at the same time. A suggestion was to progressively disable the various modules in SD untill cpu returned to normal, so as to isolate the culprit, then turn it back on and note the cpu.