new memory - detecting problem

hi,
Ive just got an old computer with Intel PentiumIII, 550MHz (5.5 x 100)
and system memory 128MB (PC133 SDRAM) changed for a new in DIMM1 512MB PC SDRAM but my computer is not detecting this new one.
Could anyone help me please - Im completly amatour and dont know what to do about it?


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