Quite easy!

Dim objWSH
Dim objUserVariables
Dim objSystemVariables

Set objWSH =  CreateObject("WScript.Shell")
'This actually returns all the User Variables, and you either loop through all, or simply print what you want
Set objUserVariables = objWSH.Environment("USER")
MsgBox(objUserVariables("TEMP"))

'This returns all the System Variables, and you either loop through all, or simply print what you want
Set objSystemVariables = objWSH.Environment("SYSTEM")
MsgBox(objSystemVariables("PATH"))

'Say you want to add a System or a User variable. Pretty simple, just use the object and assign the value!
objSystemVariables("I_AM_SYSTEM_VARIABLE") = "System Variable Sir!"
objUserVariables("I_AM_USER_VARIABLE") = "User Variable Sir!"

Remember, if you want to do this in classic ASP (on IIS 6), it will work only if you are running the App Pool under "Local System" or you have a custom account with Admin rights. It is not gonna work with Network Service!

Hope this helps!
Rahul