Extend uses space to the right side of the partition. That is why it is greyed out.
Now comes something that you will have to look out for. If you do manage to move the start of c drive into that un-allocated space your system may have issues with startup.
If you had another OS installed you could use it to both move+extend the partition and fix your boot loader(it is in the system reserved partition.). You may also be able to fix it on another system the same way.
A program called EasyBCD can edit the boot loader. You have to tell it to load one from another drive and give the system reserved partition a letter.
http://www.softpedia.com/get/System/OS-Enhancements/EasyBCD.shtml
It does take some work, but can be done.