the sensor probably is correct. this notebook is known for having overheating issues. It has been replaced twice because of an overheated graphics card. Further, it occasionally happens that the system runs dramatically slower when the graphics card's temperature is extraordinarily high. when it reached the 100 degrees C mark (or 212 F) aero glass has automatically been disabled and the graphics card shot itself down to running only the basic functions until the temperature was lower again.
is there something I can do to make it run cooler? (on a software level, I mean)
Nothing that I know of on a software level that can improve temperature levels. You could download MSI Afterburner and try turning up the fan but I don't really know about notebook cooling systems. Sorry and good luck.
Edit: You could try MSI Afterburner to get a "second opinion" on the actual temperature. Perhaps an underclock also if you don't need much graphical power. Have you installed the latest drivers from the nVidia site?
@Omnisome: I checked the temperature with MSI Afterburner. It shows the same values as SIW. The fan already is on maximum speed. And yes, I already have installed the most current driver from nvidia.
@RobisssLV: I have Nvidia GeForce 8400M GS which is a mobile notebook graphics card. Is your card a desktop pc's gpu or as well a notebook's graphics card? Mine practically never reaches temperatures as low as 60 degrees.