"Bootloader install failed"

I'm trying to install Zorin 5 32-bit as the only OS. Definitely a newbie and flying by the seat of my pants, but it was painless until...

Successfully downloaded and burned to a DVD, and the installation from the live DVD goes smoothly until a, "Bootloader install failed" screen appears, asking me to choose a different device on which to install the bootloader (neither option works), continue without a bootloader (which tells me that I'll need to manually install a bootloader to start Zorin - at this point I'm clueless!), or cancel the installation, exactly the opposite of what I want to do. Even choosing "Cancel" then clicking "OK" does nothing.

Please can somebody help - baby steps would be appreciated! If there's any useful information I've missed, please say and I'll post it.

Many thanks, Sarah.

ACER Aspire 3003
1256Mb RAM

if you try partition,check this:

Code:

http://www.linuxbsdos.com/2011/05/23/install-ubuntu-11-04-on-external-hard-disk/