If you look at the answers to this question, it may provide some pointers:
How do I install an older 2.6.37 Kernel Version?
As far as I can work out, if you go into the Grub menu at start up, just after the BIOS screen, you then have a number of options for booting.
If your Grub menu does not show when you boot up, then the simplest way to display your Grub is to press and hold the SHIFT button while booting.
As an alternative, you can always displays Grub without it booting any particular kernel:
gksudo gedit /etc/default/grub
grub_timeout to -1
finish off by running