Extend unallocated space to my `C:` drive?
The 1st partition doesn't matter. It's only the 3rd partition which sits "between" C: and the empty space.
Use something like GParted Live CD to move the whole partition all the way to the right. This will result in empty space immediately next to C: and you'll be able to use "Expand".
(GParted can also resize NTFS partitions itself, but I probably wouldn't recommend doing so for the system partition.)