This beta version is available in several format:
you can download installation ISOs on the public FTP and HTTP sites mirroring the Mandriva Linux repository. Check the list of current development mirrors on CookerMirrors. The 2007 Spring Beta tree is in /devel/2007.1/
you can also download live ISOs on the same FTP/HTTP mirrors (see CookerMirrors). *this is the recommended way to test* Mandriva Linux. Live CDs are quick to download and you can install also them on your hard drive