Memory Problems

I have a problem with my computer booting up after I have installed a new memory stick to my ASUS A7V266-E board. It had 512MB RAM (DDR) I’m trying to install another one on the second slot!! Has anybody had this problem? I’ve looked on the board manual and I don’t see anything about special settings for adding more memory. Did I miss something? People has told me to check on different slots or re-setting the bios to the default settings and nothing seems to work. Does WIN 98 SE have a problem running two memory sticks of 512MB? Please Help!!