[ubuntu] Cannot "install alongside Windows 7"

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http://ubuntuforums.org – I have a Toshiba Satellite Z930. It came with Windows 7 pre-installed and I want to install Ubuntu as well. Unfortunately the Ubuntu installer doesn't give the me option I require, which is to "install Ubuntu alongside Windows 7". It says it detects multiple operating systems and gives me the option to "erase disk" or "something else". There is actually only one operating system, so my guess is that the problem lies with the number of existing partitions: there are 4 of them. The first is the boot loader, the second is the Windows OS, the third is the recovery pa (Hardware)