Some computers have heat sensors that trigger the alarm. The actual temps that trigger it can be adjusted in the BIOS on some models. I am not sure what Sony offers on desktops.
Heat can be caused by ambient temperatures, dust, improperly functioning fans, or general inadequate ventilation in the case.
You can probably check internal temps with Everest Home to see what is happening, but you have diagnosed the problem with the external fan.
http://www.majorgeeks.com/download4181.htmlAs for solutions:
1) Check that all internal fans are working
2) Clear out any dust bunnies with a can of compressed air (
DO NOT blow into the fans themselves without blocking the blades to keep them from spinning as you can damage fragile bearings!)
3) Check temps
4) Check BIOS to see what alarm thresholds are set at, if this is possible.