LV 7.0 does not install "serpdrv" in the correct place.

[Drugar]

After installing LV 7.0 I couldn't access serial devices. I had to move the serpdrv file to the root directory of 7.0 to get it to work.

[Ray K]

Starting LabVIEW 7, serial communication is done through VISA, even when using the serial compatibility VIs.

When moving a VI that uses the Serial Compatibility VIs to LabVIEW 7.0, make sure the VI is using the Serial Compatibility VIs from the LabVIEW 7.0 and not the stock VI's from a previous version of LabVIEW. (Old compatibility VIs from previous versions of LabVIEW required serpdrv) This could occur, for example, if the application is packaged in an llb and the old Serial VIs were stored in the llb. In this case, delete the old serial VIs from the llb. When you open the main VI again, LabVIEW will load the new LabIVEW 7 serial VIs. These new VIs are wrappers for the serial VISA VIs to provide backwards compatibility.

See this Knowledgebase for more information