You do that that PS2 keyboard on that old PC, not a USB one
Whilst using DBAN to clean down some very old PCs for disposal to new homes at good causes I hit a stupid gotcha.
I booted off the DBAN CDRom but could not continues beyond the first screen as it did not detect I had pressed the return key.
Turns out the PCs were so old that, though their BIOS allowed USB keyboards (and I could enter setup and edit BIOS settings with a USB keyboard) the Linux kernel on the CDRom could not detect them. Once I switched to an old PS2 keyboard it all worked fine.
I found the same problem when putting Ubuntu onto same PCs; during the install I had to use a PS2 keyboard, but once the install completed it was perfectly happy with a USB keyboard.
Read the complete post at http://blogs.blackmarble.co.uk/blogs/rfennell/post/2012/05/10/You-do-that-that-PS2-keyboard-on-that-old-PC-not-a-USB-one.aspx