I can't figure out the exact cause, but it goes something like this:

--in normal view and toolbars (like when you first open Blender)

--render animation or single frame (in YafRay or Blender)

--when rendering is finished, click back into blender (not the console window) WITHOUT

CLOSING THE RENDER WINDOW

--try using the middle mouse scroll button to zoom (in or out)
--sometimes, it ignores that you're scrolling, and doesn't zoom, but you can still

rotate the view by clicking it
--you can still zoom with the keyboard and [CNTRL + mouse (^) or (v)]
--use the keyboard in Blender
--if it dosen't respond to the keyboard, close the render window, and then it will



**Closing the render window will not fix the middle mouse button zooming problem**

**I find that this is more likely to happen with more objects/vertices on the screen (yes, it happens in object mode too)**

I find the best way to avoid this is to ALWAYS remember to close the render window after

before using Blender again. Whenever this happens to me, I simply restart Blender.

***********My System Info*************************

Operating System: Windows XP Home Edition (Service Pack 2)
Processor: AMD Athlon
Video Card: NVIDIA GeForce MX 4000
Driver Version: 6.14.10.7189
Pointing Device: Microsoft PS/2 Mouse
Keyboard: Standard 101/102-Key Microsoft Natural PS/2 Keyboard