As whs pointed out, if you wanted to create another partition you would need to make the current D partition an extended partition - then you could create anywhere up to 24 or so extra logical drives (partitions).
But you want to install Ubuntu or Windows 8. An operating system normally needs to be installed on a Primary Partition. And you can't create any more Primary partitions on your MBR disk. So you have some decisions to make.
You could use your system manufacturer's utility to burn Recovery Disks and then delete the Recovery Partition. Or you could image the partition to an external storage drive. This would free up a Primary partition for another OS. You could still create an extended partition for data with more Logical Drives.
Lots of folks here make a System Image of the System drive and C: drive as their recovery plan. Restoring this image will bring your computer back to the condition it was in, and the way you set it up, including installed programs, on the day you created the image.
When you use the manufacturer's Recovery Partition to restore your computer you will be back to the way it was when you first took it out of the box. For people like me this is less than helpful. So I make the Recovery Disks (good for when I sell the computer), then make a system image after I have it all set up the way I want it, and then delete that Recovery Partition.
An easier, modern, and more functional solution is to convert the hard drive from MBR to GPT. In the GPT format all the partitions are Primary, and you can have hundreds of them.
To create an Extended Partition in Windows 7:
http://www.sevenforums.com/tutorials/146694-partition-extended-logical-drives.html
To convert MBR to GPT:
http://www.sevenforums.com/tutorials/26193-convert-mbr-disk-gpt-disk.html
2 important things to note:
- In order to boot from a GPT disk your computer must be using the newer UEFI (Bios).
- In order to convert the disk you must backup all your data. Everything on the disk will be erased in the conversion. You will need to reinstall Windows 7 or you may be able to restore a disk image.