Jeferson,
I no longer have LabVIEW 7.0 on my machine, but I know that in LabVIEW 7.1, DAQmx installs an example called "PID Control-Single Channel.vi." This should be exactly what you are looking for. It can be found in the LabVIEW Example Finder (Help->Find Examples) by searching for "PID."
Basically, this VI uses hardware timing (the rate will depend on your computer specifications) to acquire a single point, calculate the PID output, then output that value on the next sample clock. Please let us know if you have any additional questions!
Hope this helps,
Ryan Verret
Product Marketing Engineer
Signal Generators
National Instruments