How do I get and set Environment variables?

Use the System.Environment class.
Specifically the GetEnvironmentVariable and SetEnvironmentVariable methods.

Admitedly, this is not a question specific to C#, but it is one I have seen enough C# programmers ask, and the ability to set environment variables is new to the Whidbey release, as is the EnvironmentVariableTarget enumeration which lets you separately specify process, machine, and user.