During the initial steps, Ubuntu installer gets you to a screen where you're given 3 options for installing Ubuntu in a multiboot configuration with Windows:
Install Ubuntu alongside Windows (not a very good choice)
Replace Windows with Ubuntu (bad choice if you want to keep your Windows system)
Something else (this is what you want to choose!!!)
Once you checked the third option and pressed NEXT, you will be able to see your hard-drive partition table, and decide to keep the same partition configuration for Linux as before, includind that swap space or make minor adjustments like increasing size of your main partition for Linux, or adding other new partitions for /boot and /home.
As a beginner myself I find it convenient to choose a single large partition (around 20GB or 25GB formatted as ext3 or ext4) for entire installation, and I also add some swap space (up to 2GB in size). The one thing you should not forget is to also choose a mount point for your target partition, before pressing NEXT to start your installation. I usually choose ' / ' - root as mount point because I do not make a separate /boot partition.
You should also download Boot Repair iso image, and burn it on a CD or write it with Unetbootin on some USB stick (1-2 GB pendrive is enough for what you need). Boot Repair is a very important tool for people who decide to install Ubuntu in a dual-boot configuration with windows. Use it every time when your computer refuses to boot into either Ubuntu or Windows OS. All you need to do to fix boot problems is to restore Windows MBR with your Win7 original DVD, and then use Boot Repair CD or USB stick to fix your problems with the 'Recommended Repair' option.
On a last note, do not forget please to choose your Linux partition in the partition table, and not a different partition before pressing NEXT to begin installing Ubuntu. Also, be careful where you choose to install Grub, the so-called 'Bootloader', which is another thing that you should do before pressing NEXT to start installing Ubuntu.
Usually, Bootloader is to be installed on the partition where you first installed Windows, but I think it would be better to choose to install the bootloader on your Linux partition, and after Ubuntu installation is finished and you reboot computer, you can use the Boot Repair rescue CD to automatically fix your boot problems so you be able to boot Windows and Ubuntu.
If you choose to install booloader on your Windows partition you could find that after first reboot you can boot Ubuntu but not Windows as usual. And to fix Windows boot you need to first repair the MBR, and you won't be able to boot Ubuntu after that unless you use the same Boot Repair CD to fix your boot issues.