Right click on the "Computer tab" in the Start menu, then click on Manage.
Select the Disk Management option, then once its started find your drive in the list.
Delete the ext3 partition and then create a new NTFS partition on the drive, and assign the drive a drive letter if required.
If the drive doesn't show in the list you will need something like Acronis, or a bootable (live) Linux distro to delete the partition and then recreate an NTFS partition.