It depends of what you mean by Professional use? I'm a IT professional, and I love being able to get any Linux software that I become interested in, installed easily. Debian has more software than any other distro in their repository. I like Ubuntu on my workstations because it's more up to date than Debian. Ubuntu is a Debian derivative that has almost the amount of packages when you enable the universe and multiverse repositorys. http://ubuntuguide.org/wiki/Ubuntu_Edgy#How_to_add_extra_repositories
Synaptic is a good graphical way to install more software
Automatix is a good program to easily add greater multimedia support, and most popular apps in their latests versions.
Having Windows on the computer is a good start because it harder to create a dual boot setup with Linux installed first.
First you need to resize your windows partition, but you need to backup your important data first because there is a small chance that something bad can happen in this process.
Burn and boot the Gparted LiveCD http://gparted.sourceforge.net/livecd.php
It will run a easy to use program. Resize your Windows partition so that there is at lest 6 GB of free space after it (or to the right). Your computer may have a another partition at the beginning, leave that alone. After that is finish have the live CD restart your computer. Next run the Ubuntu setup (Or your choice of Distro)
When the setup asks how to set it up, you can tell it to automatically use free space on the hard drive. You can also use the partitioner, create a partition that is 512 MB smaller than what it defaults to. It should create a EXT3 partition that will mount as "/" next create another partition this time make sure to change file system type to "swap". The Linux installer will automatically find Windows, and setup it up as a menu option in Grub (Linux boot loader)
Here's a guild that help you with installation
After your done you probally want to access your Windows Drive in Linux
This guild can help you with that.