Ok so I want to change operating systems, I got a new PC recently that had no operating system so I installed Ubuntu on it. I now want to install Windows 7 but when I put in the install disk it says it needs my hard drive to be in the NTFS format. How can I change my hard rive to this format in Ubuntu?
It is easier to install Windows first, and then install Ubuntu. (Windows tends to like to dominate the whole drive).
What you need to do is put Windows on the disk's Primary Partition. This partition will be formatted with NTFS. you can do this by booting from the Windows CD/DVD and selecting to reformat the hard drive. Select to delete any partitions in the primary partition and then format with NTFS. Once done, install Windows OS. Then when you are done, use the Ubuntu CD to install ubuntu as a dual-boot.
If you install Ubuntu before Windows, when you get done, you will need to reinstall GRUB. If possible, the easiest solution, is to install Windows and then install ubuntu.
It's best if you do some reading before getting started so you have more knowledge of what you need to do:
It depends upon how the partition table was when you made clean install of Ubuntu. Considering default options that are available during installation of Ubuntu, the Ubuntu is likely to be installed in a single partition, which is the size of your entire hard-drive, so what I mean is that you have single partition and entire Ubuntu installation has occupied your hard-drive. So, when you boot from Windows 7 DVD, it will show you only one partition, and that will be of unknown type, because Windows doesn't support EXT4 (default file system for Ubuntu and Linux in general). And in order to install Windows 7. You have two options.
1) dual-boot Windows 7 with Ubuntu (which requires 2 separate partitions for both).
2) Wipe out Ubuntu and keep Windows only (where you can format that "unknown" partition during Windows installation).
I'd recommend you to go with the first option, and for this, you'll need to repartition your hard-drive to make room for Windows. You can repartition your drive using Gparted, instructions to resize partition are here, and then can install Windows normally.
Obviously, once you install Windows after installing Ubuntu, you'll need to re-install GRUB, since during Windows installation, the Windows Boot Loader will overwrite GRUB, you can find instructions to reinstall GRUB here.