Upgrading Ram does not work together!

Ok I am helping my parents upgrade there pc. It is running windows xp home . Its a pc133 with a max of 1gb ram memory. Two months ago I took out two sticks of 256mb and installed one stick of crucial 512mb , everything works fine,Then installed a ultra 512mb stick to make 1gb. Either stick works fine alone but when combined the pc only reads 512mb. Tried both sticks singly in both slots and both slots read ok but when combined in either combo the pc still only reads 512mb.
My question is do I need to go into my bios to change something ? Any help would be great just stere me in the right direction. Ed :iagree:

My guess, based on the limited information you have provided, is that one of your memory modules is high density and the other is not. If you ordered the Crucial from their web site go back and see what chips it uses. Probably 16x8 or 16x16, and the Ultra uses 16x32. How many chips on each module and are they double sided or single?

Check the BIOS/mobo, search for an update. Also check the manual.