System and Operating System Limitations

• Both operating systems and motherboards have limitations regarding how much RAM they can use. Most of the starter versions of Windows operating systems can only use 1 GB. Similarly, consider the limitations of your motherboard. Even if you're not over the limit, memory slots sometimes have a maximum capacity, themselves. For example, if you put a 1 GB module in a slot only designed to hold a maximum of 512 MB, it might not work.
  
  

Installation Errors

• Another possible cause is an error during the installation process. Ensure that all memory modules are firmly inserted and locked in place. Also, they typically have to be in a specific order, with the largest module in the first slot and the smallest in the last.
  
  

Bad Memory

• If your computer's second memory module is broken, it may cause this error. Switch the order of the modules; if you insert the broken one in the first slot, your PC likely wont start. Don't panic, simply re-insert a different module in the slot and return the broken one for an exchange or refund.