If you tell us the model of computer perhaps someone here can give you easy instructions.
How old is this computer? Having Win7 on this, it doesn't sound like it should be that old. Generally those batteries last at least a few years.
If it is just the battery then a new battery is usually just a few dollars, and are easy to pop out the quarter size battery and slide in a new one.
When I turn my computer on I get this message " Alert: system battery voltage is low".
I am using windows 7 on a dell.