My guess is this: your Windows setup is using some sort of LVM on your drive. So D:Work, E:Torrents and F: are not real partitions, but rather LVM logical volumes. Nautilus can see and read them (hence your 3rd picture showed them all), but the installer is instead showing you the real partition table. So, you have:
- /dev/sda1: 1MB - Windows "LVM fake partition table" maybe?
- /dev/sda2: 100MB - Windows "System Reserved" partition (created by windows 7 for many purposes - encryption, page file, etc)
- /dev/sda3: 200GB - Windows OS partition
- /dev/sda4: 750GB - Anything else (combined D:Work, E:Torrents and the empty F: partition)
As you can see, Ubuntu can read those logical volumes, but cannot install into one of them. And since you're already using your 4 primary partitions, you need to delete one to created an extended partition so you can overcome this 4-partition limit.
My suggestion on approach would be:
- Back up everything. Seriously.
- Read the link I provided you to remove the System Reserved partition.
- Make sure your Windows 7 still boots fine after removing it.
- Using Ubuntu Live CD, shrink the 200GB Windows partition to make room for Ubuntu
- Create an extended partition there. 50GB will be more than enough. 15GB if you're really desperate.
- Run the installer. Now you can create additional partitions inside the extended for Ubuntu (10~20 GB), Swap (~5GB) and, if you want, /home (whatever space you have left)
These are just directions... just say so if you need more details in any of the steps above.