As Phantaxus points out, a quick format will leave the data intact, but because the file allocation table is wiped, your PC wont be able to access the data, and will simply overwrite your old data as you use your PC. However, specialist tools can recover data that hasn't been fully erased, or overwritten, quite easily. Doing a full format will do a better job of cleaning your disk, but it's still possible for determined professionals to attempt to recover your data. It's questionable though how successful this will be. If you are really paranoid, you can get a free utility to write random data over the entire of the drive several times, making it impossible to retrieve anything meaningful. The US Government's Department of Defence (DOD) has a standard for this type of disk cleansing, and software that meets this benchmark will do a very good job of deleting your data for good. Have a look at this website for a free tool to do it :
http://www.sysinternals.com/ntw2k/source/sdelete.shtml