ubuntu 12.04 freezes after booting to desktop

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http://askubuntu.com – I can't find an answer to this problem. I have 12.04 installed, did an upgrade late one evening and when I rebooted next day Ubuntu went as far as the desktop and then frize after about 10 or 20 seconds. Alot of the advice I see on the forums assume a man has access to sudo and such tools. The only way I can successfully boot is using Rescatux. I have tried a live cd with 11.04 and 9.sonmething but that juist threw me out to a INITRAMFS screen. SO as I understand it, the grub is working ok ( although I have a quiet splash, only Ubuintu 12.04 installed). The kernel must be loading the m (HowTos)