Another tidbit from Kit George, PM on the BCL team:

In Whidbey we added the ability to set environment variables.  To see an example check out the Managed Explorer sample available at:

 

http://www.gotdotnet.com/team/clr/bcl/demos/demos.aspx

 

You’ll see there is a couple of ways to get and set Environment Variables. The key thing is, if you want to differentiate the variables, use the EnvironmentVariableTargets enumeration, which has User, Machine, and Process (default). And you will now find new GetEnvironmentVariable APIs on Environment, and new SetEnvironmentVariable APIs.