I have the same controller for a long time now, I use it for my video mixes :)
Well, at first, are you sure it still works? Try it with another software to see if your device is still sending MIDI signal. Do your vista makes some "blip" when you plug and unplug the device?
Then can you try another version of Ableton? A previous one or just the next one, sometimes few things just f*cks up everything.
Maybe you can also run another session on your computer, open a special "music" session, then put all your drivers and softwares one by one on there, all clean.
Did you check on internet and Behringer forum to see if the problem happens sometimes?
Well... last thing.... instal Win7 -_-' But you may try your device on another computer before, go see a friend, take Ableton and your BCF and try to see if it works there...
Sorry it's my only emergency procedures. I don't work with Ableton (yet), but I will surely try it soon.
PS: it's been 8 years now I work with Behringer controller and from my experience, it's very good but last max 4 years... I'm already on my third one now... Maybe it's just dying...
PS2: is it the only USB device you use? maybe your external music device takes USB too? Sure it's not a USB overflow? check your USB drivers too.