Microsoft released the pre-RC1 ISO file of Windows Vista but you'll have to be quick as only the first 100,000 people can download the file.
After downloading, please install and test the build as soon as possible. Most importantly, if there is a crash, hang, or error – please make sure that you send the information back to us using the built-in Windows Error Reporting technology (a dialog box will display automatically). Sending this information is essential to helping us improve the overall quality of Windows Vista – and collecting this information is the primary purpose of distributing this pre-RC1 release.
Once you complete the download you must burn the ISO file to a DVD. If you don’t know if your DVD-burning software will support burning an ISO file, please refer to your DVD-burning software program and search “Help” for instructions on creating a disc from an ISO file.