Unallocated means that space has no existing partition information. It is essentially blank and empty and unusable in it's current state. You must have deleted the partition--hopefully on purpose. Either use it to create a new partition or (what I think you mean you want) extend your "E" partition into that blank space to use it.
Are you looking to extend your drive E: to take up the entire hard drive? If so use a free basic version of a partition program like Paragon, O&O, or Easus-branded Partion Manager. On the other hand, try right-clicking the E: drive in Drive Manager and see if it gives you the option to extend the partition from there. I don't use Drive Manager for things like that so I don't recall if it at least has that capability.
If you delete unallocated it becomes..........., wait for it.......... unallocated. Of course you would have to create a partition on it before you could delete it because it is....... unallocated. Just use the disk management to extend the partition to take up all of the unused space, no reason to download any other apps.
Right click ->extend works in Vista and Seven IF the space you want to expand into is AFTER the current partition, not before.