Mohsin..............I suspect that you are talking PC/PC server, here...so I will restrict myself.

Windows.................go for Win2000

Now XP is fine, and I quite like it, but not as classical as Win 2000. Obviously we are talking a commercial and not a gaming environment here

I am old fashioned, so like NT4 and Win2000 in a commercial environment and Win98SE for home gaming (Yes I have ME and without problems, but you need that on a powerful machine....I prefer Win2000 at home as well, but not for gaming)

Linux is a completely different question........I asked myself:

"What would my Grandmother use, straight out of the box?".............I am experimenting with Linspire........otherwise it is Solaris.................but that is commercial stuff.

Just a few opinions/experiences