Mini 12: Ubuntu Reinstall DVD won't work, nor will Ubuntu install.

I have a Dell Mini 12 which came with Ubuntu 8.04 installed. I have upgraded to a later version of Ubuntu, but DVD playing doesn't work, and I want to reinstall 8.04.1 from the Reinstall DVD. But it won't work - it just appears to be working but does nothing. Reinstalling 8.04 from a standard Ubuntu CD fails at the partitioning stage - it seems that there is a fundamental problem accessing the HDD during the installation process - but the system works OK for normal use.

Any suggestions as to what is wrong here ?