About a year back I wrote a quick post detailing a problem (and a fix) for an issue I was having with the SQL Server Web Report Viewer on Windows 2008 Server/IIS7 (the 'RSClientController is undefined' error).
Most of the comments implied that the article helped people, however a few people weren't able to follow what was written. Partly because I wrote it very quickly at the time it was happening, and didn't grab a screenshot at the time. So I've just gone back and made a quick update to the article so that it now includes a bit more info.
Hope this helps!
Link: SQL Server Web Report Viewer Issues on Windows 2008 Server/IIS7