Most batteries are guarranteed for around 3 years.
If your car has problems startting on cold mornings wherer the engine is not turning over fast enough and you know you have used the car regularly before then the battery could be at fault.
Car batteries can be found usually at good prices from local motor spares shops.
most garages will test it for you - should take about 10 seconds - and a decent garage won't even charge for the test - it's a simple two pronged gadget that they connect to both terminals, result is immediate