Which version of Windows are we talking about? Windows well either need to install to a pre-formatted partition or it must create and format a new one during the process. If you desire only one OS you can delete all partitions with D (if its Windows XP) at the partition screen or use the delete button for all partitions in Vista/W7 setup to delete all the partitions. You want to do this until there is nothing left but unallocated space. Than you can create a new formatted partition and install Windows to it.