The temperature will rise when devices such as CPUs and Hard Disks do more than idle.
I have a Sony Pcg-Grt170 and I jsut recently started monitoring the temperature (only the HDD because I couldn't read the sensors for the CPU). It's runnning at around 51 degrees normally, but when doing a big task, the computer went up to 59 degrees, then sank back to 51 after a while. IS this normal? thanks