[ubuntu] Multiboot with Windows

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http://ubuntuforums.org – I am trying to install ubuntu on a pc already running windows7 os. I have got 3 volumes on my hard disk. 1st C drive is where windows is installed, second drive A is where I want to install ubuntu and 3rd is used for backup. When I run installation from my USB it gives my options 1. Windows installation detected install then side by side chosing between them at start#up. 2.Use entire hard drive 3.Specify partitions manually (advanced) I went for 3rd option but when I selected A drive it wouldnt let me go further. Does anybody know how could I possibly install ubuntu on my A drive only. I dont (Hardware)