It may have a recovery partition on it, are you sure it’s empty?

If you’re ABSOLUTELY sure it’s empty and want to remove it, right-click Computer > Manage > Storage > Disk Management then right-click the graphic of the D drive and delete it. The space should become ‘Unallocated’.

Now right-click the graphic of the C drive and select ‘Extend’ and follow the wizard. Under certain circumstances not all the space can be reclaimed.

Remember if you mess this up, you are in deep trouble as you have no Vista DVD.