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Yes a single computer can have multiple numbers of Operating systems in it. As such the system is aware of the fact that the boot screen requires to show options of whether to load the Windows Xp or the Windows 7. This is a very well defined approach. This approach is best seen in the absolute drama where LINUX is installed with a preinstalled Windows O S. The fact is that the LINUX must be able to identify the previous installations. And the GRUB boot loader of Linux can list the O S that is already installed in the system. Thus you nave no problems with the latest LINUX.