Once you've made (and checked - best make two copies) the rescue discs and you don't need the contents of the rescue partition any more, you can just delete that partition with a partitionning tool such as suggested by Oleh. This tool will then also allow you to increase the size of your C partition to the full hard disk size. Unfortunately, most of these partitionning programs aren't free.
Personnally, I would keep a D partition for data storage. You can make the C and D partitions both 40GB or 30/50GB for example.