I can't boot now after an update. I'm getting this:

error: cannot read the linux header
error: you need to load the kernel first

Press any key to continue...
After than it says

Failed to boot both default and fallback entries
then it goes to grub and I pick 'previous linux versions' and I can boot into -14

It's a P-III, 1133Ghz, 512RAM, Dell Laptop
I have:

3.0.0-14
3.0.0-13
3.0.0-12

It's been running Lubuntu fine for about six months now. Last night I did the updates and it won't boot.

I scanned the RAM and the hard drive and they are fine. Then I tried the 'previous version" which is 3.0.0-14 and it works fine. I went into
/etc/default/grub and changed the video to 640x480 and did 'sudo update-grub' and that worked so I know that's working.

So, it seems to me that either:
3.0.0-15 doesn't run on a P-III, or

3.0.0-15 got corrupt somehow
How can I uninstall -15 and re-install it, ... or ... where would I need to look to see if -15 stopped supporting P-III's , ... or, can I back out the update somehow?

thanks