Originally Posted by XavierP
Format the partition it sits on and remove the master boot record.
If Linux was installed the usual way so that it takes one or more partitions and installing it took over the MBR, you want to start by making Windows take back the MBR.
Do not destroy the Linux partitions before making sure you can get Windows to take over the MBR.
Depending on the version of Windows and depending on the resources available to you (such as the original Windows install CD) there are various methods to make Windows take back the MBR. The fixmbr program is the simple method in some versions of Windows.
Once the MBR is fixed, you don't want to "format" the Linux partitions, you want to delete them. You may also want to extend the ntfs partition to use the space.
If you installed Ubuntu from a liveCD, the easiest way to delete partitions is to boot that liveCD and use the GUI partition editor (I think gparted is the one on that liveCD). For old versions of Windows (XP etc.) that is also the best way to extend the ntfs partition. For Vista or newer, the disk manager software in Windows may be a better tool for extending the ntfs partition.