Leave the extended partition, that shouldn't be touched.

If the rest is NTFS, you'll want to at least partition enough space for Ubuntu as ext3 and then create a linux swap partition depending on how much ram you have. If you have 2GB or more of RAM don't bother with a swap partition. Otherwise allocate a couple gigs of space for the swap partition. You'll take all this space from the free space of the NTFS partition.

The bar all the way to the right is the swap partition. Notice how small it is compared to the rest of the drive.