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I have installed ubuntu inside windows 7 via wubi. How do i uninstall windows 7, keeping ubuntu as my primary OS?
Unfortunately, you can't, at least not easily. Wubi's designed to install Ubuntu inside the Windows partition in what is called a loopback file. Essentially, when Ubuntu boots, first the kernel mounts the Win7 NTFS partition, then looks for the file that contains the Ubuntu file system and "pivots" onto it. The cleanest way to get rid of Windows would be to reinstall without using Wubi.
Of course, technically it would be possible to manually repartition, copy everything from the loopback filesystem, delete Windows, repartition again, install the bootloader, etc. But I think that route would be nothing but a trail of tears for anyone except maybe Linus Torvalds ;).
Don't forget to back up your data!