System freezes. Can't find anything in the logs

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http://unix.stackexchange.com – My linux desktop system freezes (aptosid/debian sid) sometimes and after the reboot a can't find any infos about the reason the freeze in messages/dmesg/syslog/Xorg.*.log. When it "freezes" i can still move the mouse in X and sometimes even move the windows for a short time before the system stops responding. Last time i could even switch to the text-terminal and enter my login name, but every text-console stoppend after that and displayed no password prompt. SSH login also stops not work after the freeze. I think the problem startet some weeks (maybe 1-1.5 months) ago. I have a windows (HowTos)