On the one PC still using a HDD, I get this casper/vmlinuz file not found error.
casper is a directory in the root of the filesystem on a Live CD which contains the kernel (vmlinuz) file and will not be on an actual system installed to a hard drive.
Is this a wubi install of Ubuntu?
Do you boot with Ubuntu Grub? xp bootloader?
Do you get this when selecting Ubuntu from the boot menu?
A little short on detail.