Yesterday I planned to install Windows 7 on my personal laptop in which Linux Ubuntu was installed previously, so I created a bootable pen drive of Windows 7 because my DVD drive was not working properly and then I plugged pen drive into my laptop and when I was pressing for F12 to get multiboot system on startup then it was not showing the menu and simply starts Ubuntu, then I tried several times but it was not showing, and also I tried to get into Setup to configure boot option and was pressing F2 but still not going into BIOS setup and continue to load OS.
When I was tired I thought to remove pen drive from my laptop and then tried to press F2 or F12 and it was entering into setup and was showing multiboot menu also, then I got the problem was due to pen drive plugged in before startup, I don't know why it was happening like that.
Then finally I started my laptop without bootable pen drive and when setup was asking for password then I plugged in the pen drive and pressed Ctrl+Alt+Del to reboot and after that I pressed F12 and then it detected the bootable pen drive and finally multiboot menu displayed.
That was the solution of my problem, it may be useful for others.
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