This is daughter's laptop, and she reckons its hotter when running on mains rather than battery. I dont understand why, but have noticed the mains adapter gets fairly warm as well. Probably nothing to do with it though. Presumably, to clean with aerosol, the air-jet goes into the air outlet? - and wait for cloud of dust from the air inlet! Might try this tomorrow.
It might be "hot" because of various background programs running that shouldn't be... don't know what operating system you have, but have you looked to see what's actually running in either "reliability and performance monitor" (Vista and W7) or use task manager.
At idle displaying just the desktop it should be around 1 to 3% as a figure to aim for. Look at first picture here, click here
If it's Vista or W7 make sure you are on a "balanced" rather than high performance power plan. click here