Much depends on your usage intentions - I'll take it as corporate/office desktop... I'm a little confused by the OS you specify, because Win2K is getting on a bit, and RedHat as we knew it is toast - it's Fedora and RedHat AS/ES/WS now.
Security:
Win2K is pretty bad "out of the box". A lot of the problems aren't easily fixable unless MS provides patches. Some can be worked around, eg disabling Internet Explorer and using Mozilla, etc.
RedHat Linux is reasonable "out of the box" these days, but still wants some tweaking to nail it down. RedHat is very good with providing patches over RHN (also drawing from the various projects that actually provide the software in the distro). Linux obviously doesn't have the same drama over viruses and worms as Windows machines do (yet?) particularly if you are not running server software.
Reliability:
Both are good, in my experience. As long as users don't install random crud, it's ok. We have both here, and basically don't ever have crashes. Linux may be better able to prevent random crud being installed however, and separate user space from system space.
Usability:
Subjectively equal, I'd say. KDE 3.x and Gnome 2.x desktops are both excellent, IMO. OpenOffice and Mozilla are fine to use as well (same on both Windows and Linux). Deskops basically operate the same way, and there's even the "start menu" metaphor in the form of the Gnome Foot menu or K menu. Some users who are used to earier versions of Windows might have trouble with any transition, but no more than a WinXP move. A sensible admin will just make some shortcuts/links for common apps until they learn the menus (not hard, just some folks work that way).
Cost:
Just got silly. RedHat was free to download and $60/pa for updates but has rocketed to $180+ for a WS (workstation) upgrade service, and is about $100 for their "Professional Workstation" variant. Obviously you can save dollars by looking at other options, eg Debian, Mandrake. I don't even know what Win2K would cost anymore.
Hopefully I'm not writing your homework essay for you here, but if you have any specific questions please ask.
<i>Knock Knock, Neo</i>