One problem you've for is that C: and D: are not contiguous (next to each other). The only way to merge them would be to delete both D: and the 2GB partition (you should firstly find out ig there is anything there - it might have been a swap partition from your linux install), then you'd need to expand C: to fill the remaining space.
Another problem is that D: appears to be a logical drive on an extended partition. Generally systems can have a maximum of four partitions. To get around this, someone, somewhere decided to make one of these partitions an 'extended partition' which could host any number of partitions within. This is what seems to be happening on D:. Not a problem in itself, but you'd need to delete the logical drive first, then the extended partition.
If the Windows disk manager doesn't work, you could also try booting into a linux liveCD/liveUSB and using the partition manager (gparted) in that.
And if all else fails there might be some LVM-type solution (I think you need to convert the volume type in Windows somewhere) to logically combine the two partitions into one (as opposed to actually combining them into one)