Does the program report an error? If so, what is the code? Do you have the error clusters wired to an indicator that will report an error code or are you using the error handler? If you're not using any error hadling, you really need to fix that.
The driver necessary for serial communication is NI-VISA. That is on the device drivers CD that came with LabVIEW so you should be able to install it from there or download the latest version. You can open MAX (Measurement and Automation Explorer) and view all of the software installed and verify whether VISA is installed or not. Older versions of LabVIEW had the option of using a different driver so it would also help if you explained what version of LabVIEW you converted it from and what serial functions were used.
You should also verify that you can talk to the instrument with a program like Hyperterminal. There may be a problem with the port that is not related to LabVIEW at all.