Ubuntu can be considered as slightly newer than Debain. But I don't think that's your problem.
Kernels can be compiled with support or optimisation for specific CPUs. Sometimes that means there's a minimum specification that can be used. The minimum X86 CPU supported by Debain stable at the moment is a 486. I'm not so sure about Ubuntu.
Either it's not recognised your CPU correctly and installed a kernel optimised for something newer or Ubuntu doesn't support CPUs as far back as a 486. Although I always assumed they did.
However, that's all a little academic as we don't know what you're useing. If you've still got Debian installed then you can check what your CPU is.
As root instal lshw
apt-get install lshw
Then to get info on your CPU run;
and post the output here. That might shed some light on the problem.