Installing Vista SP1 did the trick. I somehow or for some reason hadn't finished installing SP1 and kept thinking I had, so there's the problem .... I installed SP1 and fully updated Vista, put in the last 6GB and it's working now.
I also had a problem with Vista not booting without the DVD in the drive.....but that was down to a setup error in BIOS, that had set the boot drive as my external HDD, which doesn't have Vista installed on it. Duh!
