Windows 7: problem with booting

I have an acer aspire 4736,3gb ddr3,intel core 2 duo laptop which i have been using for the past two years. Recently i observed that while switching on my laptop, it won't complete booting process at first attempt. It displays the booting screen but before the login screen appears(sometimes even after 1 minute of appearance of login screen) the machine shuts down itself suddenly without any warning. After two or three attempts, it boots perfectly. There is no problem until the next boot.I am using the present battery from last year onwards(this is a new one i got from authorized acer service center i year ago).Still i am getting a battery backup of nearly 2.5 hrs since i handle it care.I never had this problem in the last two years.Please help me in this regard.

Try doing a Startup Repair and see if it fixes anything. Also use the sfc/scannow command to check your system files for corruption/deletion.

Try the above methods if you're unsure whether it's a hardware problem. If it is a hardware problem, the above fixes won't do much.

Try removing the battery to run on A/C to see if the same occurs.