Windows XP Professional is a full-featured operating system from Microsoft. What I like about it is that most hardwares can easily be detected and installed without needing the driver cd. Installations are also done automatically.
Windows XP requires at least 64 MB of RAM, so I couldn't install it on my oldest computer, which is still a Pentium 1 with only 32 MB. It may be a bit heavier to run compared to Windows 98 (Thus, games may run a bit slower), but if you're running it on a fast machine at least Pentium 4 or higher, then that's not a problem.
Installing it is a breeze, and using it is also very easy if you're already used to the other windows versions. Updates are very frequent from Microsoft so you can be ensured that your machine and software will work at top shape in this OS.
I recommend Microsoft Windows XP Professional for use in business.