Installing the 6.0 development should be okay. It seems that as long as I've been doing LabVIEW, I've always had at least two different versions installed and sometimes three. Never had a problem except with hardware drivers. If you do a custom install, you then have the option to not install any hardware drivers. Once upon a time, LabVIEW always installed itself in the \Program Files\National Instruments\LabVIEW folder but since 6.0 I believe, it installed in a version specific folder so you shouldn't have to worry about overwriting the old 3.1 folder but I would make a backup to be safe.