I want to know the which brand of server you are using for Baan software.

http://www.baanboard.com/baanboard/showthread.php?t=43&highlight=operating+system

I would expect W2k to have picked up some since this 2001 poll, but in general these percentages don't change too much over the years.

Are you asking this question simply out of curiosity or are you trying to establish a sense of best business practice?
In case of the latter, I would not let it depend on what others are using, but more on your existing infrastructure.
When the options based on that are open, then stay with the mainstream: MickeySoft, HP or Sun (in no particular order).