logon script

I need to make a batch file that will check two things. First,if the environmental variable is being set. I know you can do this with the set command but i do not need all those results. second i need to check the hkcu\temp and tmp.
If these do not exist Iwant to be able to set them, if they do exist I do not want to do anything. what commands should I use to make this happen? should I utilize just set or can i blast it with regkey?