If you want to play with the latest Microsoft Vista build, it's available to the public here.

This page includes information to help you download a special test build of Windows Vista. This build (5728) has a number of improvements and updates from RC1, but has not been put through the same internal testing process as RC1 and therefore may be unstable in certain installations. We are making this release available for a limited time only (and only by download) in order to get broad distribution and testing in a variety of PC configurations.