How Can I Roll Back a Kernel Update?

In the Grub menu, select Previous Linux Versions to go to your list of previous kernels. The selection is just below the latest kernel in recovery mode.

To remove the kernel, execute in the Terminal

sudo apt-get remove 2.6.38.11.26

then

sudo update-grub

I hope this helps.

New kernels do not delete the older ones.
The last one (i guess 2.6.38.10) should still be there in your grub menu.
Just reboot and select that.

You might also want to install startupmanager which can be seen at fossfreedom's link, which will provide an easy GUI to change the default kernel to the working one.