Your machine is rather old (2005?) and does not support certain extensions the recent versions of Ubuntu kernel require.
From How To Install Ubuntu 12.04 On Non-PAE Capable Hardware
Physical Address Extension (PAE) is a feature to allow (32-bit) x86
processors to access a physical address space (including random access
memory and memory mapped devices) larger than 4 gigabytes.
Ubuntu 12.04 (as well as Kubuntu 12.04) uses the PAE Linux kernel by
default for 32bit ISOs so old computers that don't support PAE can't
boot the latest Ubuntu version. But there is a way to install Ubuntu
12.04 LTS Precise Pangolin on computers without PAE support: using the non-PAE netboot Minimal ISO (there are also some alternatives, see
The non-PAE netboot mini ISO lets you install the non-PAE kernel and
the desktop environment you want: you can select to install Ubuntu
Desktop (with Unity), Kubuntu Desktop and so on. One note though:
since the minimal CD will download packages from online archives at
installation time instead of providing them on the install CD itself,
you need a working Internet connection during the installation.