Oh you will like this. It requires no extra program or anything so it's really clean, but it takes a few steps.
First download this...http://www.neowin.net/forum/index.php?showtopic=140707
It's listed twice, click the second one. UXTheme Multipatcher v.40
Run that and restart.
Now your computer is ready, Go here to browse and download Visual Styles.
http://browse.deviantart.com/customization/skins/windows/visualstyle/
This is where it gets slightly complicated, but it's still easy.
When you find and download a visual style, they come zipped. A lot of times they have extra files you don't really need. The only file you need is the one with the .msstyles extension.
So just grab that sucker, and put it in a folder with the exact same name.
For Example: "vista.msstyles", put that in a folder called "vista".
Now, put that "vista" folder in this directory on your computer
C:\WINDOWS\Resources\Themes
BOOM You're done. Now you can go to your Desktop Properties (Appearance Tab) where you changed the appearance before, and "vista" should be listed there.
Now when you want new ones just go to that deviantart site above, download one, put the .msstyles in its own folder, and put that in the C:\WINDOWS\Resources\Themes folder and it should be listed. =D