There should be a generic-pae kernel in Ubuntu. Make it the default if any RAM is detected that is mapped over the 4G boundary or if the processor have NX support. BTW, see this for the real story about PAE and NX in Windows XP SP2+: http://yuhong386.spaces.live.com/blog/cns!57E2793D0C53276F!147.entry Yuhong Bao