I think you'll need to give more details!
And the kernel meta package in Ubuntu is normally called linux-386 (or linux-686). The kernel package itself is called linux-image-2.6-386 (which gets you the newest 2.6 kernel).
However, you may want to install linux-image-2.6-k7, which will be optimised for your Athplon XP, or the meta-pacakge linux-k7.
Does this help?