Kernel Oops: is this a software problem or hardware?

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http://www.linuxquestions.org – I'm running Debian testing. I last upgraded on or around Dec 11, which I think included a kernel update. Everything seemed to be working at that point (though I'll admit I wasn't reading the logs). Dec 24 I started getting Kernel Oops messages. I had the entire window system freeze up so that Ctrl-Alt-Fn wouldn't switch windows, but yet remote logins were still possible. I observed a strange behavior where images in firefox (iceweasel) would display as large black boxes. And lately I've noticed that when exim4 runs I get a kernel oops message. Now based on what I read about Kernel Oo (HowTos)