CPU Temp is rising!!

[Dimbulb]

Initially, my CPU temp used to hover around 10C above my case temp. Under idle it would be 30/41(case/CPU) at idle and around 36/51 under load.
That was one month ago. Recently its more like 33/48 at at idle.

does anyone know whts wrong?
I havent shifted my computer or even opened it up during this time.

thanks

shang

 

[Dimbulb]

Forgot to add that I'm running an AthlonXP 1700@1800 with a SK6 in an Antec sx 1040

 

[lsman]

open it up and see if there are some dust build up on fan