partition

http://www.partition-tool.com/

or just use disk manager

What OS?
Vista you can use this link that tells you how to repartition your disk on the fly.
http://articles.techrepublic.com.com/5100-10878_11-6170510.html

XP you will need a 3rd party program partition magic is good... but XP doesn't really like to be made smaller... I would suggest, back up your data, and reinstall the OS if thats the case...

If you are using Windows 7 or Vista or 2008, you can use Disk Management snap-in to resize partition. For other operating systems, please refer to ,Partition Magic software:

http://www.partition-magic.org

gparted and Easus patition tools work well

what ever you do definitly backup your system first (maybe image it)