[ubuntu] Ubuntu not booting !!

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http://ubuntuforums.org – I installed Ubuntu 11.10 alongside windows xp in C drive using a Live USB flash drive. Installation went smoothly and after installation it asked to restart. I restarted and removed the flash drive. It automatically booted into XP without showing OS boot options and did all sorts of check scans and In MY COMPUTER i saw that the 7 GB i allocated to ubuntu is gone. I restarted again and it only shows XP no ubuntu boot option. Please help !! (Hardware)