Open the case side carefully and see if you can read any of the numbers on the mainboard. Look between the PCI slots. If you can, post them here. Alternatively you can download Aida32 or Belarc system advisor as well to partially ID your motherboard's chipset and operational capabilities. Meaning the native front side bus of the mainboard. Knowing if your system runs a Pentuim or AMD processor helps, as well as the speed of the processor. This is of course, only if you don't already know what motherboard you have and i'm assuming that you don't because you posted no info about it. Any info you can give will be helpful. If you are totally in the dark with this stuff then I would get a savvy PC friend to hellp you physically ID the mainboard. Then you can properly download the motherboard manual from the manufacurers site
Aida 32
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MajorGeeks/Aida32
Belarc System Advisor
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Belarc Advisor