There are a few apps out there that claim to make your xp machine look like vista, but none come close to the Vista Transformation Pack. 

This one is the real deal though.  The real Vista Transformation Pack by Windowsxlive.net.

It's featured on SoftPedia, with over seven million downloads, and over 4000 reviews with an average rating of 4.3 out of 5.

The resemblance to Vista is incredible.  The visual stylings, the sounds, buttons, menu's, even the boot screen is the same.  They went over the top, with amazing attention to detail.

Check out this screen shot to see for yourself.. can you tell the difference between this and Vista? (click here for larger image )

There are a few downsides though, as with any major modification like this.  With all of the features installed, it will be a resource hog, similar to a Vista installation, so I wouldn't install it if you're short on Ram.  Another downside is that there are a few incompatibility issues, particularly with a Windows update (KB925902), but there is a work around for it.

Security wise, the only Adware that it contains is that it changes your home page upon installation, but personally that doesn't seem like that big of a deal.. not like it's that hard to change back.

Check out their website for specific information about troubleshooting, known issues, etc.

Go check out Softpedia to try out the Vista Transformation Pack .  I doubt you'll be disappointed.