Too much RAM

i changed my ram from 128MB and 256 MB to one of 1GB and 518MB and my computer stopped working. the cooler starts and windows is loading, but it restarts as soon as it finishes. what can i do? i don't know a thing about computers,so please make it simple...
 
What Operating system are you running and what are the specifications of your PC (motherboard, CPU, etc)?
 
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