I need some help with the Windows installer

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http://askubuntu.com – I am having some trouble installing Ubuntu 12.04 with the Windows installer. The instructions at http://www.ubuntu.com/download/help/install-ubuntu-with-windows say (in step 7) that I chose to boot with Ubuntu, but the black screen doesn't come up: it just goes straight to Windows. I've tried to boot it from a USB stick but it won't read it until Windows boots up. So can anyone help me? Some computer info: HP pavilion 520n not on the internet Windows XP Home edition version 2002 Service pack 2 Intel(R) Celeron(TM) CPU 1300MHz 1.30 GHz, 510 MB of RAM I think that's all you'll need. (HowTos)