I'm not entirely positive about this, but I'm pretty sure you can only have one version of the "NI-CAN for CVI" files in your computer, regardless of how many installations of CVI you have. This is because these files are installed as part of the NI-CAN driver, not CVI. And you cannot have side-by-side installations of the driver.
So you can only "fix" this by doing something which will remove the NI-CAN support from CVI 6.0 and add it to CVI 7.1. If that is what you want to do, you can do it by uninstalling NI-CAN from the Add/Remove Programs in your Control Panel, and then reinstalling it. When you reinstall it, it should automatically install the files in CVI 7.1 -- it picks the latest version of CVI it finds.
Luis
NI