Check in Disk Management, is the 50GB partition marked "Active'?
If it is, that's the partition with the Boot Manager. If you delete that partition you won't be able to boot anything.
Do you have the dual boot menu when you start the computer?
Is Windows 7 set as default OS?
If yes to the above two questions, follow this tutorial to remove Vista from the Boot Menu. How to Delete a Listed Operating System at Boot in Windows 7
You should now be able to format or delete the 50GB partition.