EXTRA RAM BUT NOW WONT BOOT

Hello,
Can anyone help me. My husband built my grandson a computer about 4 years ago, it had a P965T/A motherboard and a microsoft dual core processor with 1GB (2x512mb) RAM running XP home, now he wants to update to Windows 7. We went to PC World and purchased a 2 GB of RAM as per the booklet and I put it in slot 3, all we got was single long bleeps, so i took all the Ram out and put the 2GB in slot 1 all works fine, then added the 512MB into slot 2, again it all worked, but when I put the other 512MB RAM into slot 3 again we heard single lone bleeps and wont boot at all. The manual says that it is able to have 4 x 2GB into the slots available. Obviously all the different Memory work individually as have checked that. Is there something in the Bios i have to enable, i have had a look and nothing is obvious to me. I have a little knowledge of computers but unfortuneatly my husband is no longer with us to ak for help, so i'm hoping somebody will read this and be able to.