If not already, insert Flirc into USB of PC on which Flirc GUI software is installed
Navigate to flirc installation directory, in Windows XP this is normally C:\Program Files\Flirc\
In there, there is another directory called cli change to this directory (cd cli)
From this directory you can run the command line program flirc.exe
You can check the flirc is recognised by entering
flirc.exe version
To change the sensitivity enter
flirc.exe sensitivity 0
(as I said in previous post, 0 seems to work fine, you could try other values, 3 didn't work for me)
If successful you will get a message about having to reprogram the device, TBH I'm not sure if that is actually necessary so try the remote first to see if it works, if it doesn't reprogram it via the GUI
Alternatively, from the Run menu:
"C:\Program Files\flirc\cli\flirc.exe" sensitivity 0
(make sure you put the quotes in the right places)
HTH