I falsely believed it to be due to my new graphics card, because the problems started barely a month after installing it.
But I ran some tests and found out that it was my CPU causing problems. My AMD Phenom II was idling at ~60 deg C, and after running any intensive programs, the temperature was crossing 70 deg C and the PC was restarting.
It's been two years since I got this rig, so I guess my heatsink must have degraded. I want to know what kind of cooling solution to purchase for this situation.
EDIT : I would also like to know how badly I've damaged my CPU, seeing as it's been idling at 60 deg C from 2 weeks.