My PC is an eMachine T5212 running Windows XP. I replaced the memory from 2 512mb sticks to 2 1gb DDR2 PC-5300 sticks. Now my PC will not boot up. I returned the 512mb sticks and everything is back to normal. Could it be that the mother board can only 2 512mb sticks?
According to most memory sites 2gb is max. 5300 is correct memory.
Try the new memory modules one at a time, see maybe if one is bad.
If it will not work with the modules one at a time, then it is some sort of compatibility issue, try another brand of memory.