In the 100GB free space you could set up a partition as big as your memory, ie.: 4GB for swapping. then you may use the rest 97 GB for ubuntu.
You may create those partitions as primary, as IDE/SATA allows 4 primary partitions to exist.
At installation time, do set the ubuntu partition as bootable.
When asked, you may install the ubuntu boot loader to the MBR, and let GRUB manage the booting partitions. Other wise, you should configure the boot manager of W7 to learn about the new ubuntu's booting partition and this way be able to start ubuntu.
On the other hand, GRUB will let you start the OS you may like.
All that may be done at the installation process of ubuntu, when hard drive partitioning.
My advice: when in the ubuntu's disk partitioning section of the installation process, create a partition for the root - / - file system about 30GB big (this one si the one to be set up as bootable), a 4GB partition for the swap and a 66GB partition for /home, that is users directories. Use Ext4 file systems, as it is stable, tested and well known to work. Do use GRUB.