Before i even address linux vs windows in this post i will tell you, i did not bother to read all the replys as i am just gunna post my own view anyway, i myself use xp/nt/slackware/freebsd/ and slackware-live, i will just state some facts first of all before i let my opioin take over and start ranting..
NT for me is more secure than freebsd, i know alot of you will be thinking i'm on drugs for saying that, but i'll explain why! first of all we all know what POSIX is right?? for those of you who dont this will be a VERY broken down explaination
POSIX is defined as a set of standards to see what OS's could call themselfs *nix ok? well NT was rated a POSIX.1 subsystem which leaves freebsd lurking behind, i could be really bold and say NT is more *nix than fbsd, but i wont lol! granted it's windows based POSIX but from my point of view it's still POSIX and it's still granted
also NT offers finer graded DAC controls... Segregated users.. i could rant all day!! anyway i myself personaly use slackware and love it, but i love to study windows security because it's great
here are some very useful *nix based programs
Msn client -
http://amsn.sourceforge.net/msn.htm
Aim client -
http://gaim.sourceforge.net/
Yahoo client -
http://messenger.yahoo.com/messenger/download/unix.html
ICQ -
http://www.miranda-im.org/
CD burning software -
http://www.xcdroast.org/
Office utils -
http://www.openoffice.org/
Window manager -
http://www.fluxbox.org
Gimp(graphics editor) -
http://www.gimp.org/
Xchat(GUI irc client) -
http://www.xchat.org
Irssi(text mode irc client) -
http://www.irssi.org
and there is lots more, post what you want linux todo and i'll direct you to the correct software