Not enough info!
You havent given us your motherboard specs!
You havent told us how you congfigured the new RAM editions (ie did you take out the old modules when you added the 512's or do you run them in combination / What DIMM slots do they fill etc etc)
"2 sticks of Centon 512MB DDR RAM" is not enough technical info for the new modules in trying to establish compatibility.....
Run this freeware program and give us more tech info on the above requests:
http://www.majorgeeks.com/EVEREST_Free_Edition_d4181.html
Regards