The error you are getting is related to your directory not being setup up as an application in IIS. Goto Start --> Run --> Inetmgr to start the IIS SnapIn. Navigate to your website directory, right click, and select properties from the Context Menu. A properties window will appear.
Click the 'Create' button found on the lower right portion of this window then click apply. Before you click OK check to see if you have an ASP.NET tab across the top of this window (it will be the last tab on the right hand side) if you do click on that tab. The first property is Version and, by default, 2.0 should be the selected value, change this to reflect which version of the framework the directory should use.
Finally, to answer your real question: IMHO the best way to pass values between an ASP and ASP.NET application is to pass the value via query string. If the information is sensitive you could write the information to a database and then just pass a database ID along through the query string. You can not access ASP Session values in ASP.NET and vice versa.
Read this if you want to know how to get a correct reply for your question:
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