I installed Ubuntu in trial mode. Grub wasn't installed properly, so I reinstalled Windows and now it gives me the option to choose between OS. However, when I try to install Ubuntu, it says that "This computer has multiple operating systems running on it", when it's only Windows 7 and Ubuntu! What can I do? It only allows me to use it in trial mode.
closed as not constructive by Uri Herrera, Jorge Castro, Luis Alvarado♦, Ringtail, Stephen Myall Jan 11 at 10:29
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Though I am not 100% sure that, I fully understand your question, I am trying my best. I think, you were trying to install Ubuntu, failed to complete installation and then re-installed Windows. you tried again to install Ubuntu, now it is showing "multiple OS is installed".
I suggest you to choose manual partitioning mode which is labeled as "Something else" in the installation phase. Now select the partition which you selected for Ubuntu on the first attempt. And create a swap partition also. Now proceed with normal installation.
You can check this simple guide for manual disk partitioning. Though it is for 11.04, the procedure will be much similar in 12.04.
Hope this will help you.