It could be a bug in IE. Recently, on one of my PC's, all of my plugins got screwed up. I can't open PDF files directly from the browser, and instead get a white screen, a popup box that says "Adobe Acrobat" and an OK button which does nothing. I also use Skype, so it should be converting phone numbers on websites to a format where I can just click on the number and it will dial. I checked to see that the plugins were still enabled and they are. It will likely require that I reset IE to defaults and/or re-install Adobe Acrobat and Skype. So In this case I feel your pain, but can provide no useful anser to your problem.
Make sure the Adobe plugin is not disabled in IE8 would be my first suggestion.
But it's not only my site it happens with ALL sites.
Also it's not just IE8 - Firefox does exacly the same.
The opaque dialogue box with no text is just as Mike explains.
I don't usually use 'fancy' themes as them take up a lot of resouces tending to stick with the basic desk top BUT one of the features of Windows 7 is the pop up preview when hovering across the toolbar which only works with Aero themes.
I hope that this bug doesn't manifest itself into the official release but then perhaps that too much to expect - given uSofts track record.
Well, I've just installed the official Windows 7 Home Edition in 64 bit mode & guess what - the problem still persists.
OK - it might open up an embedded pdf or two BUT it soon stops showing the file.
Maybe it's my hardware but I'm not changing that, so it's got to be a standard desktop appearance, which incidentially WORKS.
Aero might look flash & without it, the much talked about features when you hover over open items on the task bar don't work, but it needs fixing 'cause I can't be the only user to experience this problem.