Batteries come with only 1 yr Standard Warranty which means their expected life is only more or less than a year. It's time to replace yours.
I have used a Dell, a Sony Vaio, a Toshiba and a HP laptop but I've only seen Mac books from the display in Best Buy, and the main point of comparison is the thickness of these notebooks. Now, there are a lot of factors which may contribute to the computer heating up. The 1st and foremost of which is to consider the processor. As we all know, the faster a computer's processor is, the more energy it needs thus the more heat it gives off (Law of Physics). So if a laptop runs on the slower versions of AMDs, Atoms, Celerons or even Pentiums, you don't even notice the heat which is minimal. But if your computer uses the multitasking processors like the Core 2 Duo, Quads or Core i-7s (Core i-5s are currently available in desktops only), then do expect it to give off more heat as you have your computer do more task. The other aspect to consider that produces heat is battery charging. Basically, as you charge up a battery, it heats up because current runs through it. Add that to the humidity and temperature of the location, it may feel even hotter than usual. FYI: I always tell my customers, a laptop's battery is similar to a cell phone's. You charge it when it's depleted and you unplug the adapter when it is already fully charge (this will prevent the battery from getting overcharged and also extend the battery life). So for less hassle, I always advice my customers to just remove the battery if they plan to use the computer for long hours straight (more than 1 hour) and just run it on AC power alone so as not to overcharge it and shorten the battery life. As you know, dust is almost always in the air. The air vents needs checking from time to time for accumulated dust might have clogged in the air passage way within the laptop itself. Use a flash light to check in the air vents. Check the bottom of your computer, it usually has rubber pads that provides ample space between the laptop case bottom and the table. For the computers that I have used, these rubber pads provide enough clearance for ventilation for the hot air. It never hurts to be cautious, so for my own laptop, I bought a $15 2-fan cooling pad from Fry's Electronics to help suck away the heated air from the bottom air vents of my laptop. FYI, laptop, as the name itself... well, I really don't use it on my lap b'coz sometimes our leg may already be blocking the air vent without our knowledge.
Also, if your computer is running a lot of applications that means that the HDD (Hard Disc Drive) is moving very fast and any minute (small) movements of your legs creates vibrations inside (which we wont see from the outside) that causes damage to the HDD discs, and eventually in a long time (when the damage is too much and in some cases the HDD makes a sound) the hard drive will crash. So, if your laptop came with a HDD protection utility, just like what Toshiba and HP has, you can always lower down the sensitivity level to level 2 or 1 but never completely disable the protection utility to prevent the HDD from getting damage from the inside and use the computer on a stable surface. Now, like what I always tell my customers, to always create a restore point (every last day or first day of the month) and make sure that you constantly back-up any important pictures, documents or files from your HDD to a Flash Drive/Thumb Drive or writable DVD±R or an external HDD, so that you will not lose any of these info just in case something bad happens.