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I have a 500 GB hard disk with three partitions.
One has XP and other has Win 7.
Can I install Ubuntu on the third?
You can install multiple number of O S in a system with a single hard disk.
The following are important parameters
1. See that the partitions in which you shall install UBUNTU meet the minimum hard disk requirements.
2. See that the LIVE session works for that particular version.
3. See that the system meets the minimum requirements for the OS.
4. See that it is a latest version that supports XP and Win7.
5. See that after installation, the GRUB boot loader will list all the available versions of OS installed in the system at boot time of the computer. The user chooses the correct one.