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#21
Follow exactly the steps given above to reseat RAM, swap sticks and try individually to see if you can get one to run correctly, then run the Crucial System Scanner software to find out what type of memory is in your computer.
Run the memtest86 CD for 5-6 passes to confirm if any or which stick is bad.
Google the exact make and model number + Memory Upgrade, read and study the results. Check against specs for the model gotten by googling make and model + Specs, and google the serial number off the RAM.
If you have a CompUSA/Tiger Direct store, you can call or go to the counter and they will normally have it in stock for a fair price. Or use the results of your research to shop online, click Shopping at the top of the Google page where you search the make+model for Memory Upgrade.