Did you check to see if your system can support more than 128meg sticks? A system that is so old it is running PC100 memory is likely to be running Win98 or ME, which both have problems with over 512meg of memory.
You didn't tell us anything about hardware/software on your system, that eliminates the ability of us to give much advice.
i just added 2 sticks of 256mb of pc100 ram to my computer the other day..but when i right click/properties on my computer it says i only have 383mb of ram...does anyone have any sort of help they could offer? thanks

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