Inspiron 1525 overheating without any reason

hi,,dude,as soon as i start my laptop,speedfanfan shows the temp. around 40 which i cannot accept as just on starting a laptop in an air cond. room the temp cannot raise so quickly in 2 seconds..how is this possible..
the other ques is when i start any heavy application even before starting the application just after clicking the temp readings increases by 10 degree..how is this possible so fast.is speedfan wrong?

also tell that the laptop fan runs when detects high cpu usage or high temperature.if on high cpu usage as i know,then what would happen if one continues using his laptop without high cpu usage for hours,then too the laptop will become hot and then willl the fans not run??