How to Reinstall Windows Without Losing Ubuntu
Ubuntu has a partitioner named gparted. You can get this by going to applications>terminal and typing (sudo apt-get install gparted) without the brackets. This program allows you to see the partitions on your hard drive and create new ones. If you are installing Windows, for security reasons I do recommend creating a NTFS partition. Have a careful look at the partitions on your hard drive. You can erase the Windows partition and re create a NTFS blank partition of a size that you'd like the entire partition to be. Do yourself a favor and try to name this partition. Otherwise, take full notes of what each partition is, what the size is and the format of it so that when you come to install Windows, you don't accidentally overwrite Ubuntu!
Now that the partitions are created, insert the Windows CD and reboot. Remember to change the settings in BIOS to allow the CD drive to boot first. Once Windows has loaded and is ready to install, select the partition you created (the NTFS one) and install Windows on there. Once the installation is complete, reboot the computer (remember to take out the CD when Windows is FULLY complete) and as the computer starts up, the grub loader of Ubuntu will load up, but you will be prompted which operating system to run.
Generally, the top Ubuntu line is the current version of Ubuntu, the second line is for recovery purposes of the current version, third is an older version which you must keep, fourth is a recovery of the old version and line 5 is a memory test. Then a new option will appear stating that there are other operating systems for example Microsoft Windows XP Professional (whatever Windows you have installed).
So if you have followed the above instructions correctly, you should have both Ubuntu and Windows on the hard drive but in separate partitions. In Ubuntu, you can access your Windows partition (which is not recommended for inexperienced users) so you can store some data on it. In Windows however, you cannot access your Ubuntu partition. So go forth and spread the multi boot word! In case you were wondering, you have just completed a multi boot installation in which you are running two different operating systems on one hard drive. It is possible to install more than two operating systems, but make sure you have the space and other requirements to do so.