how do I erase ubuntu?

The following instructions assume that Ubuntu is installed normally, as a standalone system, and not "inside" Windows with Wubi. However, if you do have a Wubi installation, or you are not sure, then please reply indicating that.

If you don't currently have a Windows system installed and you want to install Window from scratch, then you can simply use the Windows installer to wipe the disk before installing to it. Alternatively you could boot from an Ubuntu live CD, select Try Ubuntu rather than Install Ubuntu, open System > Administration > GParted Partition Editor, and delete all the partitions associated with Ubuntu. (If there's no partition on the disk you want to keep, then you could create a new partition table for the disk.)

If you currently have both Ubuntu and Windows installed and you want to get rid of Ubuntu but keep the current Windows system, then you can delete the partitions associated with the Ubuntu system either in Disk Management in Windows, or with GParted in Ubuntu. In this case, you'll also have to restore the Windows system's boot loader to your MBR. You'll also probably want to dynamically expand the Windows partition to take up the whole disk. See https://answers.launchpad.net/ubuntu/+source/ubiquity/+question/174029 (post #3) for details about this situation.

Please note that, either way, removing your Ubuntu installation will remove any files in your Ubuntu partition, so you'll want to make sure to offload those somewhere else first.