Unable to install Ubuntu 11.10 - Cd with downloaded iso file doesn't work

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http://askubuntu.com – being unexpierenced with Ubuntu, I tried to install Ubuntu 11.10 on my DELL Studio XPS running with WIN 7 Prof. Following the recommended installation steps from internet, I first downloaded the iso file, burned it on cd and restarted the system from this burned CD forced by pressing pf12. Nothing happened (black screen with blinking cursor) and after a while normal WIN 7 startup appeared. Finaly I tried the 32bit as well as 64bit version with the same disappointing result. Seems not to be so easy to install Ubuntu for me and therefore I would like to ask for any help. Thanks! (HowTos)