Fact that you had Windows 7 preinstalled doesn't mean that you don't have UEFI. You probably have the UEFI boot disabled by default.
To install Ubuntu alongside Windows manually you have to have 2 partitions

/ - root partition (your system partition)

swap - swap if you are planning on putting your computer into sleep mode (generally swap = amount of RAM you have + a bit more)

Or you can have 3 partitions The same two as described above and

/Home partition - where all your data being saved. If you do not create this partition that Root partition will be used as a Home as well. generally it means that if you will reinstall the OS you won't have a possible to save partition with the data.

Then when you decide how you are going to partition your drive you want to select where to install grub. Commonly it goes to

/dev/sda

So if you have that already selected, don't touch it, either wise change it.

And that's it, you can click Install button. Ignore the message about missing boot partition. I have been installing OS without it all the same and it runs perfectly fine.

After that you follow basic GUI installation process.

As far as I go, never trust automatic partitioning options in Ubuntu installers, as you may end up with nasty surprises. When installing Ubuntu, personally I always prepare the partitions manually. Since you want to keep a dual-boot set-up, most flexible would be to create a new extended partition and inside it create the several logical partitions.

In the extended partition I usually I reserve:

• 5GB for swap (or whatever appropriate given your RAM)
• 25GB for / which will contain all your system files (but some will say that this is overly generous, as 15-20GB could suffice given your installation habits)
• the rest for /home which will contain all your user data

I tend to use ext3 file system for the above partitions for reliability reasons (but you could of course use ext4 or reiserfs if you so prefer). Sometimes I include a /boot partition no bigger than 500MB using ext2, but I cannot remember if this is more hassle than necessary; from memory, this is where kernels and grub config files end up.

Additionally, see My approach for replacing current Ubuntu with newer for a discussion on how manual partitioning can be approached.