This error occurs because of the different versions of NI-IMAQ on your machine. To solve this, uninstall NI-IMAQ that got installed with LabVIEW 6i. Then install LabVIEW 6.1 with NI-IMAQ 2.5.2.
If this doesn't work, then uninstall all NI-IMAQ (make sure all IMAQ folders are gone), do a custom installation of LabVIEW 6.1, and choose not to install NI-IMAQ. After you install LabVIEW 6.1, install NI-IMAQ 2.5.2 separately.
Zvezdana S.
National Instruments