If you download and use the Partition Wizard Boot CD, and you really do want to combine C and F, you would have to do the following:
1) Boot from the CD
2) Copy off the contents of F to an external drive (if there is not enough room in C to hold the contents of F)
3) Remove the F partition
4) Move the D partition to the right, to the end of the new unallocated space
5) Expand C to fill the new unallocated space
6) Copy the saved Fdrive contents into C
7) Reboot into Windows