You need to provide more info. If you're going to install another Linux distribution you don't need to do anything. Just boot with the new distribution's CD or DVD and let it overwrite the MBR as part of the install.
If you have a Windows CD it depends on which version of Windows it is. For example, if it's Windows XP, boot with the CD and access the Recovery Console. From there run fixmbr. If your Windows version is very old, you may not have fixmbr, in which case you can run fdisk /mbr