I would be more concerned about the heat, humidity isn't great for them but the heat will kill them faster than anything. If you can afford a portable a/c unit or if there is a window in there you could put a window unit in that will take care of both problems as a/c units pull the moisture out of the air anyway.
If your garage isn't over 90 degrees Fahrenheit if it is your computer could really overheat and could cause a electrical fire. It also depends how long you use it. If you only use it for like an hour or less you should be fine.