The taskbar (the bar along the bottom with the start buttom on it) is what you change with visual styles. There are hundreds for both XP and Vista. Like i said before, you can change the look of yours to that of a different OS. Check my desktop screenshot in the post your desktop thread. It's Vista right? Wrong, It's XP with a Vista visual style and ported sidebar.
Now, it seems to me that alot of you are interested in all of this, but don't know what you can actually do to customise your computer, so I've thought of a few lessons for you
Lesson 1: Internet Browsers
Now, some people think that Internet Explorer is the internet itself, it's not! It's merely Microsoft's browser, and like alot of Microsoft stuff, there are alot better ones out there. Alot of people only use IE(Internet Explorer) because it comes preinstalled on Windows or because they haven't heard of any other browsers.
The two best alternative browers are Opera and Firefox.
Opera is preferred by some because you can change it's looks, it has a built in email client, a built in bittorrent client, a built in widget engine and some claim it's more secure and faster than any other browser, although results vary from test to test.
Firefox, is preferred by most for almost everything about it. You can change it's looks, you can add hundreds of extra features with it's many addons including a sidebar for all your ebay listings, the ability to block adds on webpages, extra toolbars, an IRC client, etc. and most of all, it's preferred for it's speed!
Now, I've used many browsers in my time, IE6, IE7, Maxthon, Opera, Firefox, Google Chrome. And in my opinion, and many others, the best is Firefox.
Now i urge you all to go and download Firefox, use it for just 1 day, and alot of you will never want to use Internet Explorer ever again.
I'll leave you with an example of how fast Firefox is.
On a forum i go on, when i was using Internet Explorer, if i tried to load 2 threads at once, it would take me half an hour.
Now that I'm using Firefox, i can load 6 threads at once in around 5 minutes.