Shuttle mainboard FX43G, 462 pin AMD Athlon XP 2800+ 166 MHz.
Overheating is currently the favourite suspect for frequent (daily) BSOD crashes on the above system, particularly since running it without its cover it has not crashed in 4 days.
The mainboard has three 3-pin power take-offs for cooling fans labelled 1, 2, and 3. The system / mainboard manual lists Fan 1 as "CPU" Fan 2 as "Chipset" and 3 as "System". Fans 1 and 2 are duly connected to their respective fans, but there is no fan 3, and given the small form factor of Shuttles I cannot see anywhere a third fan could be located. Fan speeds can be controlled via the BIOS with an auto setting that responds to CPU and System teperatures, user can set this to control fan1, fan3, or fan1 & 3 together. Fan 2 sits on top of the chipset and is always on.
So where is Fan 3??? One might expect a "System" fan to serve the whole box, but as mentioned above there just isn't space for such a thing (there is of course a fan in the PSU, but that has its own power supply! with no need for a motherboard take-off).
Is this just a case of a generic board used by larger systems as well as Shuttle? (what threw me here is the excellent detailed manual appears to have been written specifically for the Shuttle, and the section on cooling decidedly implies that 3 fans are used).