Memory error
The bottom line.
This could mean Bad RAM
Not enough RAM (and/or Swap space)
Conversion makes INTENSIVE use of your computers resources. It can trip up weaknesses.
If you have 1G or more of RAM , you might get and run: MEMTEST86 (it runs from a bootable CD. It is also included on all Ubuntu 'Live CD's' )
Burn your own CD with the ISO downloaded from
http://www.memtest86.com/
Let it run overnite (1 or more passes) or until you are seeing
red 