Ubuntu

For many years, ordinary computer users have really only had 2 choices of operating system:
a. Apple/Mac if you were heavily inclined towards publishing &/or were not concerned by the cost of your equipment.
b. Microsoft Windows for everybody else.

The near-monopoly position of Microsoft is by now so well known & so unhealthy that even the E.U. is reacting & taking small steps (with big-figure penalties) to unwind it.
I don't expect that action to affect the percentage of Windows users very much any time soon.

A similar near-monopoly position in web-browsers, where Microsoft's Internet Explorer had elbowed its way to the front, has taken a very big dent from the arrival & rapid adoption of a good free alternative, Mozilla's Firefox.
This success was partly due to better user satisfaction (tabbed browsing etc) partly due to better resistance to intrusion & partly due to a real philosophical desire to support "anything but Microsoft".
http://www.mozilla.com/en-US/

Similarly with Microsoft's almost universal Office suite of applications, Word, Excel, PowerPoint etc, a free alternative in the form of OpenOffice.org has appeared and is slowly building credibility & a now-sizeable customer base.
I think an honest assessment would say that these products are not quite up to the standards of their MS equivalents yet, so that the success can only be put down to economics & ideology.
Recent converts have included the French Gendarmerie & 400,000 other French Administration users, presumably more swayed by economics.
http://www.openoffice.org/

Inside this "Office" area, a small war is raging on document formats & it is probably safe to say that OOo (OpenOffice.org) has saved the world from having MS stuff a compulsory set of new document formats down our unwilling throats, but maybe that is speaking too soon.

Coming back to the main operating system, an alternative to Windows & Mac has existed for over 20 years in the form of Linux, but for most of that time it has been reserved for a relatively small number of geeks who liked getting their hands dirty (digitally speaking) & were happy to talk to their PC in its own language.
I have no competence to explain in more detail, but it seems that Linux has several fundamental advantages over Windows, which make it more robust to intrusion & allow it to run without anti-virus & firewalls.
There are countless articles explaining the advantages of Linux & describing the associated concepts of open-source software and community development.
In any case, the result is rapid development of apparently efficient, stable, flexible & free operating system & applications.

There have been several attempts to propose Linux to a wider public, but they have usually stumbled because of the steep initial learning curve required to deal with something which is not Windows.
Now, at last, one of these efforts – Ubuntu – seems to have concentrated enough expertise & money (sponsored by Mark Shuttleworth, South African millionaire Space Tourist in 2002) to develop & promote a version of Linux which is user-friendly enough, complete enough & well-enough supported & documented to be feasible for the general public.
http://www.ubuntu.com/

I started using it in 2006, in parallel with Windows XP on the same PC.
At that time, there were still big problems of compatibility with some hardware (printers, scanners, wireless…) but the situation has improved steadily in that time, whilst still not being 100%.
Ubuntu is provided as a free download (or a free CD if you can't download) which you then burn to a CD & can try out, running from the CD with no commitment & no risk.
That will show you that actually using it does not require any significant geekiness.
Be prepared for some things to be different and, after acclimatisation, better.
Assuming you like it, the CD then includes everything required to install the Ubuntu Linux operating system on your PC, either alongside Windows (strongly recommended) or instead of Windows.
Obviously you should read & follow the instructions carefully!

Once it is installed & connected to internet, you can revel in the disconcertingly-named Synaptic Package Manager to browse a list of thousands of optional, free, officially-validated, downloadable packages which will let Ubuntu do just about anything you can imagine.
Clicking on any package brings up a list of everything else you might need to add, remove or update to make it work properly & further clicking gets everything downloaded & installed in one go.
If you wonder, for instance, which of many possible PhotoShop clones you might prefer, well try several, they are all free, then wipe out the ones you don't want, leaving Synapt (Synaptic Package Manager) to do all the dirty work of checking compatibility & clearing up afterwards.
This is a real pleasure.
The same Synapt keeps track of available updates for absolutely every package on your PC & will regularly offer you the latest approved versions.

The main Ubuntu system gets a programmed upgrade every 6 months, which will be offered by Synapt & which you can apply as soon or as late as you like in its 6-month window.
On the negative side, I have found that such upgrades sometimes bring new problems, and for this reason I currently keep 2 versions of Ubuntu available on my PC.

There is an abundance of official & unofficial information & help, and an astonishingly lively & helpful forum.
http://www.ubuntu.com/support
https://help.ubuntu.com/
http://ubuntuforums.org/

Ubuntu will run on much smaller & older PCs than Windows Vista or 7, needing only 30% of the RAM size & 25% of the disk space, for instance (You might need to confirm those off-the-cuff figures).
In addition, there are special "light" versions (Xubuntu) which will run on next to nothing, if you want to play on a very old PC you have lying around.
https://help.ubuntu.com/community/Installation/SystemRequirements

Is it perfect?
- Of course not.
Is it as easy to get going as the Windows that came pre-installed on your PC?
- Of course not.
Does it work as well as Windows?
- Probably better in many or most ways, with a warning about some hardware compatibility.
Is it improving regularly?
- Yes, definitely.
Is it worth trying?
- Yes, if you are willing to put in a bit of effort, especially at the beginning.
- Yes, if you would enjoy feeling like part of a team, instead of like an unwelcome nuisance or a gullible sucker.
- Yes, if you prefer free to expensive.
- Yes, if you would like practically unlimited choice in how to do things.
- Yes, if you want to do your tiny, insignificant bit to squeeze Microsoft back to a useful size.

Parting thot: "Computers make it easier to do a lot of things, but most of the things they make it easier to do don't need to be done." - Andy Rooney