This shouldn't be too much of a problem. Things will work best if you boot from a LiveCD (or LiveDVD), choose to Try Ubuntu, and once it reaches the default Ubuntu desktop you can open the Dash and open GParted. (You're going to be doing things with partitions, which is best done from a Live desktop so the drives are unmounted.)

First, you can shrink sda3 to free up some space. I would suggest that you not mess with sda1 and sda2; Windows will likely not like that and might respond by making it difficult to get back into Windows. Similarly, don't move sda3; just shrink it in size by the amount you want to give to Ubuntu.

You can then expand the Extended - sda4 - partition into the free (unallocated) space you created. Then you can expand sda5 to the left to grab the available space.

You might find that you'll get an error message telling you that you can't do it because some drive - probably sda6, the Linux Swap partition - is still mounted. If that happens, you can right-click it and unmount it without causing any problems, and then just proceed.