If you have a second partition on your primary HDD,
first move the starting point of that second partition
using PartitionWizard, and remember the amount
by which you "shrunk" that second partition:http://www.partitionwizard.com/
When that task is done, then run this same software
to increase C: by the same amount that you decreased
your second partition.
We just did this on an older PGA-478 socket PC,
and it worked perfectly the first time.