my PC boots properly then after 15-20 minutes it shuts down on its own. I have checked the power supply and it is ok. i've also checked the processor fan and it is also ok. what could be the problem?

sounds like its overheating ,if you have a addin video card ,check its fan,and also maybe a motherboard chipset may need to remove cpu fan/heatsinc and add some heatsinc compound .

also go start /run , type in ,eventvwr.msc
check system errors may show something .

Could be 1 of one million things like -
- Power supply is insufficient to run all devices. Remove external devices like usb drive, phone, external HDD ... still nothing? disconnect cd/dvd device....Still nothing?...

Have you upgraded Hardware recently?
- a new video card?
- more Ram?

Did the computer shutdown unexpectedly due to a blackout?
- your bios settings may have been reset to default and Windows doesn't like the default settings.

Are you Gaming online?
Are you gaming at all when the shut down occurs?
Are you watching YouTube Videos online?
Are you doing anything with Videos eg: watching, editing, compiling, decrypting?

What are you doing at the time of shutdown?