Laptops are notorious for generating a lot of heat -- to the point where they slow down and shut down or have errors. Unless you live in a refrigerator (or at one of the Antarctic research stations), it happens in almost all environments. Some places are harder on laptops than others -- any dry dusty environment like West Texas or Arizona get hot enough to cause more frequent problems. Hidden dust in the household, such as from having a cat or dog -- or living in Arizona AND having cats and dogs) mean that you may need to deal with this problem quite a bit. And there are just some machines run hotter than others -- for example, the Apple Mac Book Aeros and the IBM T60.
So... you've noticed that things have heated up and are wondering what to do about it. First thing is to put your hand near the vents on the computer and see if the fan(s) are actually blowing air out of them. If the answer is, "yes but it's awfully hot", that's still a good sign. If your answer is "no, I can't feel any fans blowing" then take it to a repair shop (unless you happen to know how to build computers -- and in that case, go replace the fan.)
The next thing to try is seeing if you can fix it without taking the case off. There's a number of simple checks you can run to see if this cools your laptop down.
First, shut down some extra programs. I'm notorious for having 3-4 PDFs open plus a lot of web pages plus World of Warcraft and so forth. Do you watch lots of videos? Listen to streaming music? Try closing a few things down and see if your machine runs cooler. If this works, then consider getting more RAM for your computer.
This is also a good time to run a program to clean up your registry and your hard drive. Efficient computers run cooler.
Next, take a look at what you've got connected to your USB ports. Things like external hard drive and network dongles put a load on your CPU and can cause your system to run hot. Instead of leaving them permanently plugged in, try plugging them in only when you're using them. USB hard drives cause a lot of heat problems in laptops.
Luckily, your mouse won't contribute to this problem... but oddly enough, some of the laptop external cooling fans will cause the system to heat.
Now take a look at WHERE the laptop sits. It needs to be on a hard surface (like a lap tray. Many laptops have cooling vents underneath and flexible surfaces (like your legs) obstruct the vents and cause overheating. Check to see if any of the labels that they put on the computer have slipped and started covering a label.
Next thing to do is check and see if your laptop has the power saving functions enabled. To find out, use the search function on your computer to find out where your particular brand of computer has them hidden. Make sure that the power saving functions are enabled.
Finally, buy a can of compressed air and blow it through the vents and in between the spaces of your keyboard. You can also purchase a laptop cooler board -- basically a small board with one or more fans -- and maybe USB ports. They come as both USB versions and external power versions. The USB ones are more convenient to travel with, but external power versions are much easier on the laptop's boards.