Ok, then you are not using the source code you are using the pre-compiled deployment package.
Most likely cause is if you did not install the files into the correct folder or if the site is configured for the wrong version of .NET like 1.1 instead of 2.0
or if you don't have .NET 3.5 SP1 installed (this is confusing because 3.5 is a superset of 2.0 and the setting in IIS still will say 2.0, it will not say 3.5)
Hope it helps,
Joe