PostMessage() on Vista64 behaves weird

Question

The scenario is like Process A invokes certain function foo in Process B, that actually does a PostMessage() on Process C.

Process A invokes the function foo when then there is a WM_KEYDOWN message.

This works just fine on all 32 bit platforms, but on Vista 64 - when there is a single keydown a burst of events is generated and there are like a bunch of PostMessage calls happening.

Note that all the processses are 32 bit that running on vista WOW64.

Any help is appreciated.