For you XP heads like me that aren't ready to give up a solid operating system for one riddled with bugs and such, there is hope for you yet. 

Object Desktop can do the trick

Object Desktop is a full blown suite of products that can make your XP machine more like Vista.  Higher quality icons, better visual styles, better sounds, the whole thing.  Check out my Object Desktop review for more details.

Use the Vista Transformation Pack

The Vista Transformation Pack has a lot of stuff that can make your XP install resemble Vista.  They emulated everything from visual styles, to buttons, menus, sounds.. you name it.  Check out my Vista Transformation Pack review for more details. 

Vista looking themes

CrystalXP.net has what they call BricoPacks, known as themes to the rest of us, that can make your XP system look and feel much like Vista.

Now these aren't straight themes like other windows themes, they change some windows files in order to enhance some effects.. but don't worry, they are safe, and they remove easily.. restoring the operating system back exactly the way it was.

They come in a few different flavors, for different tastes. Check them out over at CrystalXP.net. They have plenty of screenshots to preview before you download. Oh, and did I mention that they're free?

Vista Wallpapers

There are a lot of vista wallpapers out there, but here's a post I found that has a lot of good ones all in one place.  Mix up some of these with a nice looking theme, and people will swear that you have Vista installed.

I've also created a post with some links to nice vista wallpapers that I've found around the net.