Cleaning the system would have been my first suggestion. LabVIEW 6i and IMAQ Vision 5.0 should be able to be used together. I would recommend trying it on another machine that hasn't had LabVIEW or Vision installed on it before because sometimes junk can be left behind if the uninstaller has problems and it is hard to find all of the registry keys hiding in there. I assume that it is something specific to that machine and that reinstalling the OS would probably resolve the issue, but I know how unappealing reinstalling the OS is so I would try another computer first.
Cheers,
JR A.
Application Engineering
National Instruments