I wanted to use dual-monitor desktop wallpapers on my dual-monitors, but couldn’t find any reasonably priced (OK, I really mean free) software.  Then, I came across DisplayFusion from Binary Fortress Software!  (for Windows 2000, XP and Vista) I’m using it on my Vista machine and luvin it, get yours now – it’s free:

 

They explain it even better than i can, but i will say that this is great software, and the pro version is on $15.00!
DisplayFusion Free Multi-Monitor Desktop Wallpaper Software

DisplayFusion will make your dual monitor (or triple monitor or more) experience smooth and painless. Advanced multi-monitor wallpaper support, Flickr integration for More >