Purple blank screen on quitting installation or shutting down

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http://askubuntu.com – I just bought an HP Pavilion m6- 1045dx laptop with windows 7 pre-installed. I am unable to install ubuntu alongside windows, so I am considering replacing windows completely. But since I have not been given a windows installation CD, I am concerned that ubuntu may give me the following trouble that it is giving when I select "try Ubuntu" during installation: All versions of Ubuntu that I tried (10.10, 11.04, and 12.04) would either freeze to the purple ubuntu background, or as in 12.04, to a blank screen. I corrected the blank screen by selecting nomodeset in the beginning. But when I try to (HowTos)