I thought Vista did this but I'm mistaken. MS added the function into Win 7.

I found a few with a Google search for Vista Wallpaper changer but beware of suspicious web sites. Using Firefox with WOT enabled, (Web of Trust), I found some red-spotted links that I wouldn't go near.

Also, I found one at CNET's Download.com here; http://download.cnet.com/Automatic-Wallpaper-Changer/3000-2336_4-10560884.html but looking at the reviews I couldn't recommend it myself.