Back up the data, remove the partitions with the disc management tool and then create one partition and format it.
Cost a bit? Partition Magic. This is supposed to save you the data backup step but I've seen it wipeout so I can't say I trust it.
i have a Dell dimension 4100 that is running windows xp. i have an external hard drive and i just put a new hard drive in it that used to be in my old computer. the problem is that it was partitioned so it thinks its two different drives. how do i format it to thing that it is one dive again?