Installing ubuntu-13.10-server-amd64.iso on an HP Proliant ML115 G5. MD5 hash checks on the downloaded ISO.

I boot, confirm the pre-selected English language by pressing the Enter key, get the main menu. I first selected the CD-ROM test (by pressing Enter) and it confirmed that the burned CD-ROM was good. But then the Enter key no longer works (i.e. I can't select "Continue") and have to re-boot.

Selecting the Install option (by pressing Enter) the next panel is the installation language, pre-selected with English. Again, Enter doesn't work and after pressing it no key works and I have to reboot.

This is a reinstallation so I booted the existing (12.04) system off the disk. Both Enter keys work perfectly. (It's a USB keyboard with a Return key in the middle block and one labelled "Enter" in the numeric pad area to the right).

Am I doing something wrong? It's difficult to believe because to that point you can't really do much! I've googled without finding anything. Any suggestions on how to debug this, or get round it somehow?