I shoot medium format to a laptop that runs on Windows XP.
I then process on a desktop that's Vista X64.
The laptop has Hasselblad Flexcolor (16bit) installed.
The desktop has Flexcolor X64bit.
For white balance, I use a basICColor grey card.
During capture, I take a white balance so that any obvious cast is removed - purely for convenience really.
Back on the desktop, I take another during processing as a matter of course.
I'm pretty sure that I'm clicking on the same place on the card and the card is spotless. But, I do get slight differences in the reading. e.g. the Temp/Tint might be 5950/7 on the laptop and then change to 5850/4 on the desktop.
Does the OS have any say in the White Balance achieved?