Since Parallels Plesk Panel supports both 1.1.x and 2.0.x versions of the .NET framework, it is possible to choose the version used by your domains on a per-domain basis.
To change the version of .NET framework used by a domain:
Alternatively, you can go to Domains > domain name > Web Hosting Settings (in the Web Site group) and select the .NET framework version in the Microsoft ASP.NET support drop-down menu.