IE7 and Firefox behaving differently

I am trying to implement a feature for my web site for library access. Normally users access the site with http://www.famnet.net.nz. This => a welcome/logon page, you may decide to register, etc etc. I programmed access from http://www.famnet.net.nz/library.aspx?userid=testlibrary. This URL (or one like it) is intended to be a link on a web page within a library: clicking the link opens the site with the user already logged on as "testlibrary", with some differences in function compared with an individual logon. This all works perfectly in my testing, using Firefox. However it does not work with IE7. I programmed diagnostic statements that should appear from all paths: but these don't appear with IE7 (but they do with Firefox). Where do I start looking for the cause of this problem? Thank you, Robert Barnes ...