Need help on How to reformat an hp pavilion xt963 desktop?

I need help to reformat my desktop I went to this site (http://www.ehow.com/how_6026_format-hard-drive.html) and I went to the storage part and when it says First you must delete any existing partitions on the drive you are going to format. Do this by right-clicking on the partition's box and selecting "Delete Partition..." well it wont let me the delete partion is not clickable so im stuck I just want to reformat the desktop please any help??