I had the same problem.
I found an article on Microsoft that help me to down-grade
to a previous version of Internet Explorer.
(I started with "Method 5: Remove Internet Explorer
Components", and continued from there.)
The article is
at: "http://support.microsoft.com/default.aspx?scid=kb%
3ben-us%3b303399"
(Microsoft Knowledge Base Article - 303399)
What is the reason for doing this? If it is just to let some backward
site recognize you "the most efficient way to downgrade" from IE6
to IE5x could be to alter your User Agent string so that you fake it.
E.g. in RegEdit find data MSIE 6 and change only that to MSIE 5