'+' symbol problem in URL in IIS 7.x

We are sending an HTML encoded string in the Query string. It was working fine on IIS 6 (windows 2003). We have recently moved the website to Windows 2008 (IIS 7.x). Since the move any Query String that contains "+" sign i.e., "%2b" gives error on the server "404 - File or directory not found."

Any help?

Best regards.