Ram upgrade, windows 98 wont start

I have 512mb ram on a P4 2.4, just upgraded by another 512mb to 1024. However, when I put the 512mb chip in the computer windows wont start! I’ve tried using both 512mb chips seperatly and know they both work so obviously its a software error (and my botherboard is capable of taking the ram). I dont want to wipe my HDD though so any ideas? oh, I’ve tried using msconfig to stop all the programs from running on bootup but I dont even think the bootup gets far enough to start running any programs anyway. I’ve also checked my autoexec and thats not running anything odd.