You can run both versions on a machine and choose the version in the Application Pool.

That said you can most likely run your .NET 2.0 application under 4.0 without changes. .NET 4.0 is a superset of 2.0 and there are very, very few incompatibilities. 99% of 2.0 applications will just run under 4.0 only better - there are many optimizations in .NET 4.0 that make apps run better so it's a viable option to try out IMHO.

+++ Rick ---

>Hi,
>
>My customers are using my ASP.NET application on their Intranets. The application runs on .NET 2.0 and is typically installed as site on their web server (usually Server 2008). I want to add some new modules as separate applications (that will have a different DLL) but I would prefer to start doing this in VS 2010 and use .NET 4.0. Therefore, on the same site I would have some .ASPX pages using DLL built in VS 2005 with for .NET 2.0 and other .ASPX pages using another DLL built in VS 2010 for .NET 4.0. Will there be a conflict? Anybody sees possible issue with this? TIA.