Reply to VSTs - 32 or 64 bit?

I may sound a bit retarded now!

I have Windows 7 64 bit on my PC. I run Ableton, which is obviously a 32 bit app.

If I install a VST, do I install the bit version of the host or the bit version of the OS? That is, would I isntall the 32 or 64 bit version of the VST?