Installing Ubuntu in a partition made previously

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http://askubuntu.com – I bought a Toshiba laptop with two partitions: C: with Windows 7 and D: with my data. I want Ubuntu 12.04 be installed in D: but when I try the computer shows me C: divided in two parts. If I choose Something else and select /dev/sda3 a message appears: There is not a root file system. What can I do? (HowTos)