How do I extend the size of an NTFS partition?
Windows 7 "Extend" feature works only if the unallocated size is to the right. Yours is to the left. You will need to use a 3rd party partition application (PartedMagic/GParted).
Before messing around with partitions, backup everything first.
From my experience with dealing with partitions I would suggest 2 options:
GParted - a free linux distribution which can be operated from a live CD and hence removes the limitation cause by manipulating partitions from within the system. - it has very thin GUI but helpful.
EaseUS Partition Master Professional Edition - An amazing tool which can do almost everything and resize/merge/delete partitions without any problem. I really like this program since you dont have 'step outside' of windows, you just give the orders and then it does everything by itself.
Good Luck.