in general terms, I would proceed as follows...
We need to clarify some starting assumptions:
- your level of Linux experience
- your tolerance for your winbox to be completely broken
- whether you have a backup of all your data..
- if you know whether your system starts up with UEFI or BIOS
- purpose for Dual-booting. Virtualization might meet your needs more effectively.
ASSUMING you have a complete backup, or don't care about your windows system...
first, create a boot repair CD/DVD from here:
This is your first safety net.
At this point, I would boot up my winbox (with access to the internet) using a bootable Ubuntu installer CD (choose "Try Ubuntu"), and make a big-enough partition to hold your linux install. The tool you would use is gparted. If it's not included in the bootable DVD image, you can install it by opening a terminal window and typing
sudo apt-get update && sudo apt-get install gparted
you can launch gparted from the terminal window by typing
sudo gparted, or you can launch it directly from the GUI by pressing the Alt-F2 key-combo and typing
gksu gparted in the resulting dialog box
make a note of the partitioning information (I'm making some assumptions here). In gparted, choose View -> Device information. Is the partition table msdos or GPT? Also note the size and descriptions of the existing partitions (RIGHT click on each partition and choose information). How much free space did you create?
Make the partition 20 GiB for simple testing, and some growth. Make it a primary, and format it as ext4. Label it as something descriptive like "Linux-take1" (no quotes). You can make it bigger if you know what you want to do. If you need more disk space later, it is pretty trivial to repeat this process and grow the disk, or add more partitions.
Make another partition, I typically make it 8 GiB (size of RAM). Make it primary, and format it as linux-swap. label it as "SWAP" (again, no quotes)
After applying your changes, right click on the "Linux-take1" partition and flag it as boot.
At this point I would simply mount the "linux-take1" partition somewhere in the filesystem of the Ubuntu boot CD and rsync one of the many instances of linux systems in my archive to the target partition, then boot with the boot-repair-cd to make my new system bootable, then carry on with my new dual-boot system. I don't install from scratch very much (except for a new LTS release). I tend to configure a known system for a client's needs, then back it up as tarball for future use. I'll assume you don't have this level of infrastucture ;-)
So at this point (after creating and flagging the partitions), I'd reboot and make sure your Windows install still boots. If not - this is your first chance to test your safety net (did you check your backup before you started?).
Next, boot up with the Ubuntu dvd you've been using and choose the install option.
You need to NOT follow the automatic partitioning path, and at this point I need the actual Ubuntu installer interface in front of me to tell you the exact steps (and name of the option for manual partitioning). Essentially, you want to choose the 20 GiB partition you've already created as the install target, and the 8 GiB partition as the swap. No partitions need to be created to do this -- you just need to select them, then hit the go button. The installer should take care of the rest.
If you're not in a hurry, I have a winbox that needs a re-install of win7, and I can complete the detailed steps. It's a good opportunity to make sure you have appropriate backup.