At the last PDC, Microsoft presented it's vision for Longhorn, the next version of Windows. At that time, the idea was you'd get loads of cool new stuff (whizzy presentation layer, a to die for file system, etc) by upgrading to Longhorn. It turns out that Longhrn was a bit of a tall order, and the requirement to upgrade to Longhorn was something many corporates were uncomfortable with - many of them still had not fully deployed XP yet, let along wanting to think about another roll out a couple of years down the line. So last summer, Microsoft had a rethink on the scope and direction of all this technology, the result of which was that WinFS was removed from Longhorn and is meant to come later. Additionally, MS announced Avalon (the new presentatiuon layer) and Indigo (the communications layer) were to be backported to Windows XP.
As reported over on Slashdot Microsoft has now released an initial beta of Avalon. Dubbed a Community Technology Preview (CTPs), it's a huge 261MB download. This was first made available to MSDN customers in November 2004, Microsoft has now made this a lot more widely available. CTP releases are not always super stable, so if you do decide to install it, you should probably avoid installing it on your primary workstation(s). In order to use the Avalon CTP, you also need a beta version of the .NET Framework Version 2 (download from Microsoft - it's 24MB!) your XP system (which can be XP Tablet!) needs to be at SP2, and you probably need a DirectX 9 capable graphics card as well.
You can read the slashdot reader's comments over at Slashdot. As usual, there's loads of anti-MS stuff and rather inaccurate comments, , but some good information. And for details on the release itself, head over to the MSDN Site and look at the Avalon November 2004 Community Technology Preview article. And for the low-down nitty gritty on WinFX, see the WinFX On-line SDK.