I was using 512mb ram along with 1 gb ram, and there was frewuent problem of blue screen of death, after removing 1gb and with 512 alone there is no problem.

Then how can I upgrade my ram.
Mine is lenovo thinkcentre with pentium d processor and 512 mb ram

Need to make sure the ram is the same type as one installed already

Can maybe check if supply the model # of computer with mfg website - need to check what type is installed in the system and what is usable for upgrade

I can say this:

Some systems need identical sizes because of interleaving.

Some systems require the smaller ram in the largest numbered bank.

They must meet or exceed the speed. Look for a -xx at the end of one of the memory chips. What are they?

Memory could be defective.