A blog post at Tech Republic provides links to two ways to try Windows Vista for free. The simplest is to try a business version online at the Windows Vista Test Drive site. This runs in a browser window, after you install the (ActiveX) Virtual Server VRMC Advanced Control. Of course, it doesn't tell you anything about Vista's performance.
The better but more cumbersome alternative is to install Vista in a "virtual PC" on your hard drive, where it won't affect the rest of the stuff you do. As Greg Shultz points out: "Microsoft has created a 30-day evaluation VHD (Virtual Hard Drive) containing Windows Vista Enterprise Edition that you can download and load into either Microsoft Virtual PC or Microsoft Virtual Server."
The drawbacks are disk space and time:
Besides a Microsoft Virtual product, both of which are free as well, you'll need to have at least 10 GB of free hard disk space and at least 2 GB of RAM. You also have to have a very fast Internet connection or a bit of time on your hands as the Vista Enterprise VHD is comprised of three very large files -- two 700MB files and one 160MB file.
The sizes make it easy to burn them to CD.
Once you've done the download, you have to do a real Vista installation, which generally takes half an hour.