Bios recognizes all the ram but Windows does not.

I'm not sure what else I can try.

Today I setup my old Dell Dimension 1100 desktop for my mother I did a fresh XP install and all works well except the pc is not showing or using all the ram that is installed.

In the PC I have a 1Gig stick and a 256MB stick and in the Bios all the ram shows up, but when I check My System with windows it shows 256MB. For troubling shooting I pulled the 256 stick out and tried the 1Gig unit and it still only shows 256. I tried both slots with each stick and no matter what configuration I use all I get is 256. The odd thing is the bios shows 1280MB.

If either stick was bad the PC would not have booted. All the bios settings are correct, but for some reason I can not get all the ram to show.

I dont have any graphics cards or any other add on's this is a very basic unit. It has all the XP updates and drivers installed. When I turn the PC on it stays on the Dell bootup screen for a good 2 minutes or so, which it did prior to the fresh Windows install. I'm wondering if there is something wrong with the bios, but at this point I really dont know what else I can do. I know all the sticks and slots are good.

Edit: I forgot to mention when I was using the pc about 6 months ago before I got a new one all the ram was showing and working. The sticks are 2700 and 3200..........if that matters and the system can handle 2gigs max

bios > os install mode
maybe

I have the exact same problem, with the same computer, with the same Ram...what did you do to fix the problem?