ubuntu doesn't boot without flash drive

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http://askubuntu.com – i just installed ubuntu 11.04 onto this netbook. I had to use a flash key . during the install , i tried putting ubuntu on a separate partition , but it kept showing a 'no root file system is defined'. i didn't really know what i could do to fix it , so i decided to install it alongside windows. i have a windows 7 installation which works perfectly fine. So , the installation goes through perfectly . i give both OS's 40 Gigs of space . The comp restarts and NOTHING !. The computer boots directly into windows. During the install it said i'm supposed to be prompted at boot, and nothing happened (HowTos)