Why can't I extend the C: drive on Vista? Because I have Free Space to its right instead of Unallocated Space

The problem is that you can NOT extend a primary bootable partition. I think your best bet will be to boot from a BartPE CD or a GParted CD, and do the re-size from there.

"No need to boot up GParted or anything else but your Windows installation.

Use diskpart.exe's extend command as suggested by others.

If that however doesn't work, Dell has made a small utility do extend partitions on-the-fly. Even works on Windows XP/2003 boot partitions.

http://www.dell.com/support/drivers/us/en/19/DriverDetails/DriverFileFormats?c=us&l=en&s=dhs&cs=19&DriverId=R64398"

"To shrink or expand partition size, use built-in Computer Management.
But on vista/win7, drive C: can't be expanded unless there is an adjacent unallocated space next to it.
To do this,
Shrink drive D:
Make D: and E:, then delete drive D:
Now you can expand drive C:"