The type or namespace name ‘Workflow’ does not exist in the namespace

Today when I tried to open MS CRM 4.0 plugin registration tool in VS 2010, as usual it asked me upgrade the solution and when I opened after upgrade, after adding web reference I tried to build project and I got below error

Error      1              The type or namespace name ‘Workflow’ does not exist in the namespace ‘System’ (are you missing an assembly reference?)   C:\Users\Vikram\Desktop\pluginregistration\AssemblyReader.cs          75           43                pluginregistrationtool

 Then I checked .net version in project properties, and it was .net 2.0, so I changed .net version to 3.0 and it worked like a charm.

Hope it will help somebody !!!