Your system does not meet the minimum requirements to install Ubuntu. As JoeSteiger suggests, you can try installing a more light-weight distribution, however, from looking at their minimum requirements, it looks like Lubuntu may be a better choice for your machine.
- 700 MHz processor (about Intel Celeron or better)
- 512 MiB RAM (system memory)
- 5 GB of hard-drive space (or USB stick, memory card or external drive but see LiveCD for an alternative approach)
- VGA capable of 1024x768 screen resolution
- Either a CD/DVD drive or a USB port for the installer media
- Internet access is helpful
To install or try Xubuntu within the Desktop/Live CD, you need 256 MB
of memory. Installing with the Alternative CD requires only 64 MB.
Once installed, it is strongly recommended to have at least 512 MB of
memory, but you can run with 256 MB too.
A Pentium II or Celeron system with 128 MB of RAM. Desktop CD should
not be used to install on systems with less than 1 GB of RAM. Please
download the alternate ISO.