Party,

Both these will repartition your C drive to a larger size without disturbing anything on them.


Partition Magic [costs money]

Gparted [free]
Still does a good job and relatively easy to use but an Open Source Tool - GParted is just as graphical, nearly as easy to use, possibly a little faster (subjective judgment only), comes in Live CD and USB Versions and best of all it's free You do need a decent download speed to download this onto disc.

http://gparted.sourceforge.net/