Hi,
If you are referring to a windows partion you set up, XP for example do the following.
Go to Disk Utility - click partition at top of list on right hand side > select Partition TAB > Find the xp partition at the very bottom of big rectangular box then scroll down as it is not yet visible, only mac is.
Now select and press the minus button underneath. and follow the prompts. Mac made adding and deleting partitions extremely simple but forgot to add this to the help menu.