Hello. On z87 motherboards (not one, many) I have problems with Intel SpeedStep on XP64 OS.
All EIST options and C-states in BIOS are on, but after loading OS CPU runs always at maximum speed with turbo boost. Theres no change when CPU is really idle. On Z77 chipset everything is ok, cpu slowd down to 1600mhz.
Latest INF update installed.