first you need to convert you c drive to a dynamic disc, then you can extend it.
http://msdn.microsoft.com/en-us/library/windows/desktop/aa363785(v=vs.85).aspx
if for some reason you cannot delete the partition you could try to delete the data on the partition first that may help
click start in the run box (or in xp click run) type cmd
if the partition is d type cd d:
now it should look like d:Open Command Prompt.
Type:
diskpart
At the DISKPART prompt, type:
list disk
Make note of the disk number of the disk from which you want to delete the partition.
At the DISKPART prompt, type:
select diskn
Select the disk n from which you want to delete the partition.
At the DISKPART prompt, type:
list partition
Make note of the number of the partition that you want to delete.
At the DISKPART prompt, type:
select partitionn
Select the partition n that you want to delete.
At the DISKPART prompt, type:
delete partition
http://technet.microsoft.com/en-us/library/cc738489(v=WS.10).aspx