Desktop is 4 years old, problem is recent without any hardware changes. CPU now overheats to the point of shutdown when running Vista, however when running Xp (I have 2 Hard Drives with Vista on one and XP on the other) or when in BIOS the CPU temp stays or returns to normal (49c). Case and fans have been cleaned (they were VERY dirty). Using SpeedFan and temps correspond to those in bios.