If you have physical access to the machine (and it's capable of booting from CD) you might do a net-install with Etch sources. The hazard off course, is loosing your data while doing so...
One safe way is to change the sources.list back to Etch (Testing) and run aptitude to find and select the latest available kernel. According to the official Debian
package list for Testing, the latest kernel-image is 2.6.15-8.
<edit> you may want to download the kernel-image by hand and install it with dpkg, if dependencies come up you could easily get the required packages from the same source and install them at the same time.</edit>