Try using MSIBlast. We use MSIBlast to remove NI software off PCs when the MS installer/Removers fail to clean up after themselves. You can also do a Deep Clean on the PC, after having run MSIBlast restart the machine, then run Regedit (MS Tool). This will allow you alter and remove Registry entries, search for "National Instruments" and delete these. Only attempt this if you feel very confident to work within the MS Registry, it is worth while making a back up of the Registry before making any changes.
Enclose MSIBlast
Regards
Joe W (NI)