I purchased a USB to RS232 Serial cable so I could hook up my GPS unit to my laptop and found out that installing the driver on Vista 64 was problematic.
Disabling driver signing verification with the following command at an elevated prompt, followed by a reboot, enabled me to install the driver.
BCDEDIT.EXE /SET NOINTEGRITYCHECKS ON
Download the prolific XP 64 bit driver (installs on Vista 64 after disabling driver signing verification) from:
Link