HI,
Most likely you have the traditional driver for the hp34401a.
If that is the case, you need to remove the hp34401a folder located in the LabVIEW\instr.lib directory. Removing this folder should be enough to remove the traditional driver.
If you have a older version of an IVI driver, go to C:\VXIpnp\WinNT\hp34401a. In this directory there should be an uninstall executable.
Hope this helps.
DiegoF