Wubi for Windows 7 on Sony Vaio is not Working

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http://askubuntu.com – I have installed Ubuntu using Wubi on my Windows 7 machine (Sony Vaio). Everything installed perfectly, but whenever I reboot my system there is no Ubuntu (there is no dual boot menu). I checked my Windows setting and it seems there is only Windows (it could not recognize Ubuntu). Could anyone help me how to solve this problem. (HowTos)