Stardock has released the next version of WindowFX which will help give your XP machine some Vista-like effects. Here is what WindowFX 3.0 will do to your machine:
Effects include shadows under windows, min/max/close/open animations, window dragging effects, window scaling, window tiling, alt-tab task switching options, taskbar preview thumbnails, and much more.
The software will cost you $19.95 and if you use it in conjunction with WindowBlinds then you can get a real feel for how Vista will be. Or, if you so desire, you can test Vista yourself.