Which Beta did you download Bifta? There was a problem with a version that was released at the begining yesterday. They uploaded a different version early this morning to replace it. Some form of corruption issue with the first file uploaded for release.
I'd read about the Acid2 test failing as in IE6 but I haven't seen people reporting the same number of problems that you are having. Explains why it's still in a clsed Beta though I suppose.
Would the un-install instructions for XP IE6 work for IE7? You might be able to remove the registry markers and then re-install IE
http://support.microsoft.com/default.aspx?kbid=318378