Oh my goodness, where to start...
One of the greatest beauties of Linux is that you can build a stripped-down version, and for certain dedicated tasks, it will completely whup the *censored* of any Windows flavour you mention, for speed, security and stability. The other significant pull is the huge variety of software, including high-grade server spec stuff that is available for free. I installed a system for a friend the other day, based on
Ubuntu, and when I was done, it had something like 12,000 different software packages installed. All completely free.
(Incidentally, if you decide to go for Ubuntu, I recommend choosing the KDE version,
Kubuntu. You will prefer KDE to Gnome, I'm almost certain.)
Disadvantages of Linux:
A lot of stuff is done on the command line. Configuration can often involve editing plain text files. There's an awful lot to learn, and you will miss the Windows GUI-for-everything way.
Support for drivers is often more limited in Linux than in Windows.
The various components that make up a typical Linux desktop are not integrated in quite the same way as Windows. With Windows, you have only one windowing system. With Linux you have many different possible systems. Different programs may be made for different systems, resulting in things not coming together completely seamlessly. For example, the clipboard facility is in many ways not up to the Windows standard.
Microsoft Office (etc) cannot be expected to run on Linux. Hence there is a lot to learn in terms of new software. For everything.
The Open Source model means that rogue hackers can read crucial system code in an effort to find vulnerabilities to exploit.
Advantages:
Speed. You can configure Linux for speed, much more easily than Windows. If you have an old laptop you want to use, which is not up to running XP, put Linux on it. Sorted.
Security. There are VERY few Linux viruses out there in the wild. Not for the want of trying, on the part of virus writers. Linux generally has a more secure permissions system which makes it hard for viruses to propagate.
Flexibility. You can use the same base Ubuntu system as a power desktop, a gaming platform, an office workhorse, a file & print server and/or a web/database server. There are usually no licensing restrictions, so you can throw as many users at the system as the system can handle.
Cost. You can download hundreds of different Linux distributions for free. You can download hundreds of thousands of GOOD Linux applications for free.
Variety. Your choice is not simply XP, Windows 2003 Server or Windows 2003 Small Business Server. See any Linux distribution website, and you will understand what I mean.
The Open Source model. A benefit in itself; there are hundreds of thousands of people around the globe, working endlessly on Linux. Security holes are patched almost immediately, and typically turned around to end users in a fraction of the time it takes Microsoft. Also, "open source" means that everyone can view the code, and it is harder for bugs and undesireable design "features" to go unnoticed and undocumented.
I'll leave it there for now. Fairly sure GX1_Man will have PLENTY to add.