One of the new features of Windows 8.1 Preview is lets you boot your Windows to desktop natively. If you prefer to boot to the desktop directly here are few steps to enable Boot to Desktop:
1. Right click on the taskbar, then click on properties
2. Click on the Navigation tab, then under the Start screen section, check the box next to "Go to the desktop instead of Start when I sign in."
3. Click OK to close the properties window. The next time you boot Windows 8.1, you'll go straight to the desktop.
If you would like to boot to the Start screen with Windows 8 Live Tiles, just repeat the above steps and unchecked the box next to "Go to the desktop instead of Start when I sign in." and follow by a reboot.