Just a thought, but is ReportViewer configured for asynchronous rendering?
This link has some details: http://msdn.microsoft.com/en-us/library/ms252090.aspx
With AsyncRendering = true, your report will be displayed inside an IFRAME. It can by default use a certain height until it is displayed, and IE does not recognize that after the fact.
Try setting it to false - if the problem and asynchronous rendering are not an option for you, it might be possible to programmatically set the initial height of the ReportViewer low enough to avoid these scroll bars.
IE definitely allows only one type of scrollbar (at least IE8) - you can try by resizing the window on the Google homepage and making the window tall but narrow.
Stefan mohr
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