Disk could not be partitioned?

When I try run boot camp, first of all an error message comes up saying the apple software for windows 7 cannot be installed because the software is not currently available from the update server. I then un-tick that box and leave the 'Install Windows 7' box ticked. But when I've moved the scales around and decided how big I want my partition to be, it starts saying it's partitioning my HD and then displays an error message which says, "Your disk could not be partitioned. An error occured while partitioning the disk". Why did this happen and how can I fix this?