Nunol: This is a workaround to use a computer with CD drive, but without DVD. It won't help if there's no CD drive present.
You can make a 3GB FAT32 partition on your harddrive using GParted (from Ubuntu Software Center), and use Unetbootin to make a Live 'HardDisk Install' on that partition (Select Hard Disk in the 'type' drop-down box). This will make a fake DVD on your harddisk, which will be added to the boot menu of Ubuntu.
Note that this is not a real Mint install, it's a Live DVD: Live means that it's to be used to install, or to try out Mint. It will not save any changes you've made. We'll now install a real Mint out of it.
When the Live Mint is made you can select Mint in the Grub bootloader of Ubuntu, and then you will boot into the Mint Live DVD which is in fact your FAT32 partition.
From that fake-DVD you can run the Mint installer and then install Mint over Ubuntu. When that is done, you can wipe the FAT32 partition with the DVD and run this command to remove the Live DVD from the boot menu:
This command will remove all partitions that don't exist anymore from the boot menu.
After the FAT32 partition is away, you get some 3GB unallocated space on the disk. You can create a partition here if you want, and use that for anything you like.
Note that not being able to boot from USB usually means: USB boot is turned off in the BIOS, or the harddisk comes first in the boot sequence. If you know how to change this (often described in a laptop's manual) you can boot from USB.