Recently the forum is flooded with people that are experiencing troubles with the Buspirate v3.6. Pieter in the forum was no exception. He first tried the Buspirate on his Windows 8.1 and Vista machine and went to the forum after finding out hise buspirate didn’t respond to anything. The supportteam in the forum suspected the PIC was not programmed or counterfeit FTDI chips were used. In the mean time Pieter tried the buspirate on more different machines and was dazzled it did work on a Crunchbang (Debian flavour) workstation. He investigated it further and tracked the error down to the latest FTDI driver (2.08.30), after de-installing and installing an earlier version of the driver (2.08.28) (scroll down to find the older version).
More info about the process on his blog and please report if this also works for you.