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.
Have you tried running any programs under XP ?? and does the CPU temp rise when it starts running it --- THe CPU is probably just running a few more processes under VISTA than XP so is actually using the CPU slightly more but once XP starts to run a program or anything else you'll see the temp rise there also !
Probably need to get some new Thermal paste and uninstall the Fan\heatsink - clean everything well and add the new paste and reassemble - If the fan was very dusty chances are some made its way under the heatsink and is causing heat to have a hard time moving from the CPU to the Heatsink for disappation.
49C is a fairly high temp while the CPU is just at idle while in the BIOS under any circumstances so looks like there is definitely some cooling issue so best to remove everything and reassemble correctly to eliminate the problem since it is sometimes hard to narrow down the cause unless you fix everything that could be the cause.