SQL 2005 temp directory

Hello all, I have a sql 2005 server and the c:\ drive is running out of space. The maintenance plans use c:\temp and the space rapidly decreases until all the space is gone and the plan fails.

I want to change the temp directory to reside on another drive but am not sure how to do this as I don’t have much experience with sql 2005. How can I change this in Sql and will it require any down time? Can the dbs stay online?
Any help would be appreciated. Thanks.

Steve