Which build are you using? It works fine for me in IE8 (in W7 and Vista), but if you are using a beta build of IE8 from one of the earlier W7 builds that may be causing a problem.
I've never actually seen that error before, and can't replicate it at this end. Can you try disabling all addons (Tools > Manage Addons) to see if it's a plugin/toolbar causing this problem? If that fixes it, you can add them back one by one to see which one is causing it.
If that fails, see if compatibility view works on the page.
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