The command
path=dir1;dir2;dir3 will set the PATH environment variable. However, that only endures until the command window closes. In the scope of LabVIEW you would use System Exec to issue a command line action (e.g., "cmd /c path=dir1;dir2;dir3"). But, you would need to do whatever you want to do within the command process that gets executed by System Exec. You can do this by putting your actions in a batch file and call the batch file instead. If you want to persist the change you need to use something like setx.exe which is part of the
Windows XP Service Pack 2 Support Tools.