You can install any linux straight from the iso image rather than burning it into a CD/DVD and then installing.. This is how you do this

Before starting, There are mainly two things needed to install an -inx flavoured os . one is a kernal (will be vmlinuz in case of ubuntu) and another is a RAM disk or initial root file system (initrd.gz in the case of ubuntu)

Step 1:  Download the iso file from website and store the image file in a    directory

# mkdir ubuntu

#  cp <image file which youhave downloaded> ubuntu

Step 2: Extract the kernal file and RAM disk file from the iso image and store in the directory. You can use any archive manager present in the os. Better use File Roller. These file will be in the /casper/ directory.

Step 3: Add an entry to your grub i.e. in the file /boot/grub/menu.lst

title Ubuntu Install
root (hd4,0)  //Here replace hd4 with your hard disk partition
kernel /ubuntu/vmlinuz
initrd /ubuntu/initrd.img

Step 4: Save and Reboot. The option for installing will come on your grub. And
When You start installing you have to select the source as Hard Disk.