Windows NT is 4.0 version
Windows 2000 is 5.0 version
Windows XP is 5.1 version
Windows 2003 Server is 5.2 version
Windows Vista is 6.0 version
If you have a slower computer, definitely install Windows 2000.
Windows 98SE (Second Edition - USB support, not the initial Win98) - not bad, but prone to memory leaks, so you have to restart more often or it will freeze. Also, important point - Win98 works with max 256 MB of memory, more efficiently with 128 MB. Adding more ram can actually slow down the computer.
Windows ME - some people didn't like it and still preffered 98, but I had a decent experience with it. Felt like it was a bit faster than Win98 and had system restore and lots of updated drivers.
Windows 2000 - remember using a beta version of it first. Very stable for its time. Still preffered by many today. Runs well on older laptops and computers. Better memory management, so you can install up to 4 GB of ram and NTFS system supports files bigger than 4 GB. Highly recommended for older computers. RAID, iSCSI and accessibility support. Less fluff than WinXP. Can be run stable for long periods of time.
Windows XP - Stable platform (I run it sometimes nonstop month or two without a reboot). More fluff on desktop, but it can be turned off in preferences to speed it up, so it will basically look like Windows 2000. Multithreading, improved device support, kernel tweaks, better power management with hibernate, compatibility mode, fast user switching and ClearType for TFT/LCD monitors. Windows Genuine Advantage checks for valid license, but this can be easily circumvented.
If you need any help with Windows, PM me.
Mark