|If you just installed the OS on them I recommend you just do things over. You can create multiple partitions when installing WinXP of create just one that doesn't use all available space. Then later the remaining space can be partitioned and formatted.|
If you MUST do it now then you need a partitioning software. There are a couple of free ones but I have no working knowledge of them. I have used Partition Magic before.
There is always a risk of things going sour when repartitioning so you should backup before attempting, which you should already be doing anyway.
Are both of these drives in the same computer? Did you install the same OS on both? If so, why?