I assume your actual disk is much bigger than 20GB and that you have other partitions on it.
GPARTED is a pretty good and safe re-partitioning tool (download an ISO file, burn it to a CD and boot off it).
Other strategies (which you may have implemented) is to install applications to a different partition (so the OS is on C: applications on D: user data on E
You can set up shortcuts etc to the various virtual drives. If you do move applications to a different drive number, in most instances you will have to uninstall and then re-install them - still a lot of faff.
If you do decide to re-partition, although GPARTED (and most other reputable partition editors, such as partition magic) is safe, you should back up any critical first.