How do I know if my laptop has a virus or just needs a replacement battery ?

How long has it been since you physically cleaned your laptop? It could be so plugged up with dust that it heats up very quickly. Laptops inherently run fairly warm to begin with. If you don't use "canned air" to blow through the vents on the sides, back and bottom, the dust just keeps being drawn in through the vents and settles on the small cooling fan blades and make the fan very inefficient. What you described really doesn't sound like a virus. Take the laptop to an area where you can hold the computer over a soft place in case you drop it, and blow the dust out of the inside through the vents. You can also gently tap the computer while holding it at an angle to kind of dislodge the dust and dirt out of the vents. If the fan seems noisy, it probably needs the blades cleaned desperately. This procedure is probably best left to a computer repair shop unless you have a friend competent in opening up a laptop computer. This is pretty much a bi-monthly or quarterly procedure because of the constant dust/dirt environment that a laptop usually operates in. Note: If you keep the dust cleaned out of the inside, it's less likely you would need to to have the laptop opened and the fan physically cleaned. Clean it up then run your anti virus and see if there's a difference in the operation and the temperature.